Players now accumulate points by winning races in the normal first-person driving mode, called A-Spec mode. Each race event can yield up to a maximum of 200 A-Spec points. Generally, a win using a car with less of an advantage over the AI opponents is worth more points. Points can only be won once, so to win further points from a previously-won event, it must be re-won using a car with less of an advantage over the AI. There are also the 34 Missions which can yield 250 points each. Despite this, A-Spec points are experience points, not money.
The new B-Spec mode puts players in the place of a racing crew chief: telling the driver how aggressively to drive, when to pass, and mandating pit stops (by monitoring tire wear and fuel level). The speed of the time in the race can be increased up to 3x, allowing for Endurance races to be completed in less time than would take in A-Spec mode. The 3x feature, however, must be turned on after every pit stop because it resets to normal time. The game manual says that the player may speed up B-Spec mode by up to 5x, but this is believed to be a typo. B-Spec points are given out for each race completed in B-Spec mode. This increases the skill level of the AI driver in the categories of vehicle skill, course skill, and battle skill. Players can thereby use B-Spec mode in harder races as the game progresses, but this mode cannot be used on wet, dirt and snow courses.
Another new addition to the game is the Driving Missions, which are similar in experience to the license tests, but award successful completion with 250 A-Spec points and 1000 or more credits. Each mission takes place with a given car on a given track or section of track, and a given set of opponents. There are 4 sets of missions: The Pass, in which the driver must overtake an opponent within a certain distance; 3 Lap Battle, in which the driver must pass 5 opponents over the course of 3 laps; Slipstream Battle, in which the driver must overtake identical opponents by way of drafting; and 1 Lap Magic, in which the driver starts with a significant time penalty against much slower opponents and must overtake them all in the space of a single lap. Completing each set of missions earns the player a prize car. There are a total of 5 prize cars available to be won, they are the DeLorean DMC-12, Jay Leno Tank Car, Pagani Zonda Race Car, Toyota 7, and the Nissan R89C.
A new Photo Mode is included in the game, which allows the player to control a virtual camera, taking pictures of their cars on the track or at specific locations, including the Grand Canyon. This game is able to produce a selection of screenshots with variable compression rate (Normal/Fine/SuperFine) and size (up to 1280x960 72dpi), and the user can choose to save or print to a supported USB device.
Compared with Gran Turismo 3: A-Spec, graphics are greatly improved with more detail on cars and tracks (despite running on the same PlayStation 2 hardware). The physics are also greatly improved, with the major upgrade that cars now experience body movement, such as pitching (forwards and backwards rolling) under braking. Barriers have considerably more friction to slow down the cars in GT4 (in an attempt to stop the use of "wall riding"), but there is minimal friction between cars, so the advantage obtained by running into the side of another car (instead of braking) is still present. Each of the Driving Missions and Special Conditions events give a 5-second penalty for hitting the walls or opponents' cars in this way, where the car's speed is restricted to 50 km/h (31 mph) until the timer disappears when it reaches zero.
GT4 supports Dolby Digital surround, 480p/1080i (NTSC only) and widescreen modes, but 1080i is only supported in single player races. Despite the lack of online gameplay, GT4 does support use of the PlayStation 2 Network Adapter, which can be used to communicate with additional PS2s to create a multi-screen setup. In addition, the Network Adapter can be used to play games on a local subnet for up to six players, though player customized cars cannot be used in a LAN game. Support for the Logitech Driving Force Pro and GT Force steering wheels is continued from Gran Turismo 3: A-Spec. Other "PC" steering wheels previously (and unofficially) supported in GT3 were explicitly disabled for GT4. New support is given for USB storage and print devices used in Photo Mode.
GT4 continues in its predecessors' footsteps by offering an extremely large list of cars; the PAL version, for example, features 721 cars from 80 manufacturers. There are differences in the car lists between the different GT4 regional versions, and some cars have different names, e.g. the JDM Toyota Vitz is known as the Toyota Yaris in places such as Europe and Puerto Rico and the second generation Mazda Demio is known as the Mazda 2 in the same places (PAL version only). A non-Japanese example is the Opel Speedster which was sold in the United Kingdom as the Vauxhall VX220. The Nissan Z-car is known as the Nissan Fairlady Z in the NTSC-J version. As was the case with its predecessors, several well-known sports car manufacturers such as Porsche, Ferrari, and Lamborghini do not appear in the game. RUF, a company that produces cars based on Porsche chassis, but is classed as a manufacturer in its own right, reprised this role for GT4.
Some of the cars in the game are multiple variations on a single base model; there are 20 different Subaru Imprezas and Legacies, 25 Mitsubishi Lancer/Lancer Evolutions, and 48 Nissan Skylines, including the Nissan GT-R Concept. One vehicle, another Skyline, is the pace car from the "Guide Lap" licence tests and is also a prize car. There is also the GT Edition, which was the pace car without the pace car lights, and even more power (541 horsepower, the standard version packs 276). Each vehicle model has over 5000 polygons.
Car prices range from about 2500 credits for basic 1980s Japanese used cars up to 4.5 million (450,000,000 in the Japanese version) credits for the top end (mostly Le Mans) race cars. Some special prize-only cars (such as the Pagani Zonda LM Race Car '01) are not visible in the vehicle showrooms, and a few do not have corresponding dealerships, and thus are unmodifiable, for example, the Formula Gran Turismo (Formula 1 car). Also, some concept cars featured in the game ended up becoming mass production cars, such as the Suzuki Swift.
GT4 is responsible for a few vehicle firsts in the Gran Turismo series. It is the first to feature pickup trucks, such as the Toyota Tacoma and Dodge Ram. It is the first game in the series to feature the DeLorean, using the stage II spec engine (developed in 2004, hence the 2004 designation). It is also the first in the series to feature a diesel-powered car, the BMW 120d. A special edition of GT4 featuring the 120d (and the rest of the 1 Series line), and three tracks were provided to BMW customers who purchased their 1 Series automobile before the release of GT4. While Gran Turismo 2 did have a one-off F1 engine version of the Renault Espace, GT4 was the first of the series to feature a production minivan, the Honda Odyssey (JDM version). A first-generation Mitsubishi Pajero Paris-Dakar rally car, a winner of the 1985 rally, makes an appearance as the first SUV in racing trim. It was also the first in the series to feature D1 Grand Prix tuned cars such as Ken Nomura's Blitz ER34 D1GP.
The game includes some prize cars of historical interest, such as vehicles from as far back as 1886 at the dawn of the automobile. A special car called the Auto Union V16 Type C Streamline, built in 1937, can only be used in Power and Speed (which tests the performance of vehicles) or a test drive in Nürburgring, but has around 542 HP. Even some modern cars with complex body shapes cannot be raced against opponents, such as the Caterham Seven Fireblade. In Arcade Mode, these cars can be raced against a single opponent; this is the case for any convertible with the top down.
There is also a special car which is specially tuned and returns from the previous game, called the 'Shuichi Shigeno' version of Toyota AE86, which is taken from Initial D, and is licensed by Toyota, although Toyota has never used the name of the author of Initial D. Another Initial D car, the Sileighty, makes another returning appearance. Comedian Jay Leno, an avid car collector, is listed in the game as a manufacturer; one of his custom cars, the Blastolene Special or "Tank Car", is included in the game as a prize car, available after beating missions 11-20. The 2022 Nike ONE has Morse code on both sides of the car. They read "www.phil-frank.com", the website of the artist commissioned to design this car for GT4, and "PFD", the initials of the artist's company. There is also Morse code visible on the inside of all four tires that read "PFD" when mirrored.
GT4 retains all the familiar tuning parameters from the previous games in the series, but also allows weight to be added to the car. This can be positioned to affect handling or used as a form of handicapping. Another new vehicle tuning addition is nitrous oxide injection. Also, GT Auto can now install a rear wing on some cars, making it possible to adjust the car's rear downforce, which was previously only possible on racing cars or, on the first two games in the series, cars with the racing modification performed.
The game features 51 tracks, which are divided into 4 groups: World Circuits, Original Circuits, City Courses, and Dirt & Snow. Many of the tracks are new or modified versions of old Gran Turismo favorites. Notable real-world track inclusions are the longest circuit Nürburgring Nordschleife, Suzuka Circuit, Twin Ring Motegi (with three Road Course configurations, as well as the Super Speedway) and Circuit de la Sarthe (Le Mans). There are also tracks modelled after world-famous attractions such as New York City's Times Square, Hong Kong, Hôtel George-V Paris, and the Las Vegas Strip. The Hong Kong course is located in the Tsim Sha Tsui district, which, in its clockwise configuration, starts at Salisbury Road, passes through the city's waterfront and then Nathan Road.
The Città d'Aria course follows actual roads in Assisi, Italy. The race starts/finishes in the piazza in front of the temple Minerva. Just before the start/finish line on this course, written on the tarmac is an inscription in broken Italian: "Dio lo benedice — fate il suo guidare il più sicuro e divertirsi", which, when translated, means "God blesses him; make his driving the most safely and to have fun." On the Seattle circuit, the Kingdome, the previous home of the Seattle Mariners, is visible next to their current stadium, Safeco Field. Kingdome was demolished in an implosion on March 26, 2000. Because the Seattle circuit was created for GT2 in 1999, before the Kingdome's demolition, it has been left unchanged and the Mariners' now completed home, Safeco Field, remains under construction, not to be completed until 2002. (GT4 reuses the Seattle map from GT3), A section of the Opera Paris course passing through Place de la Concorde was traversed by Claude Lelouch in the short film C'etait un rendez-vous.
There are no changes in weather or time conditions during races. Even during the 24-hour endurance races, there is no transition from day to night or between different types of weather. All of the tracks are run in dry weather, with the exception of the Tsukuba Wet Race.