Need for Speed 4 Cheats
To enable the cheats you have to download the Cheats Enabler. 
Then you just enter the cheat you want in the main menu.
acar  Bonuscar
altiers Open All Tiers
bcar Bonuscar
buy Free Purchase (career)
cars All Cars
ccar Bonuscar
dcop Bonus Hot Pursuit Car
ecop Bonus Hot Pursuit Car
fcop Bonus Hot Pursuit Car
gates Cash Increase (career)
gofast Upgrade Engine (arcade)
madland Super Human Opponents (arcade)
monkey Upgrade Automatic Transmission (arcade)
moon Low Gravity (arcade)
outmyway Blows Computers tires
resetya Resets All Computer Car
tp 00-15 Drive As Traffic Cars (arcade)
tracks All Tracks
up0 No Upgrades (career)
up1 1st Upgrade (career)
up2 2nd Upgrade (career)
up3 3rd Upgrade (career)