The Denso Sard Supra is definitely the best 4-cylinder in the game.
After looking around kideng's database, only ones that are very interesting are:
Impreza Sedan WRX STi: 1270kg, 555 HP, Turbo 3, 4WD, power/weight = 0.44
Impreza Rally Car Prototype: 1230kg, 549 HP, Turbo 3, 4WD, power/weight = 0.45
S2000: 1240kg, 560 HP, NA 2, FR, power/weight = 0.45
Honda Integra Type R: 1080kg, 491 HP, Turbo 3, FF, power/weight = 0.45
Lancer Evolution VI RS: 1260kg, 580 HP, Turbo 3, 4WD, power/weight = 0.46
SuperAutoBacs Apex MR-S: 1125kg, 512 HP, Turbo 2, MR, power/weight = 0.46
Impreza LM Race Car: 1150kg, 965 HP, Turbo 4, 4WD, power/weight = 0.84 (engine type isn't actually listed)
Castrol Tom's Supra: 1100kg, 941 HP, Turbo 4, FR, power/weight = 0.86
Denso Sard Supra: 1100kg, 970 HP, Turbo 4, FR, power/weight = 0.88
Those are all the highest upgrade after being broken in with an oil change.