Graphics - For some reason the software mode in NFSIV runs pretty badly even on modern PCs, so I would only advise to use it for career and tournament runs. For ILs, simply use Glide wrapper.

Roof cam - NFSIV has more cameras than NFSIII and one of them is roof cam, which is probably the best camera for driving fast.

Damage - Damage in NFSIV is a bit random. Hitting a sign costs you anywhere between 0-3%, which makes grinding this category for lap records a bit frustrating. You can of course slow down below 100kph and hit a sign, which usually costs you 0% damage, but it's really annoying to do so.

Broken gears - NFSIV has even more broken gears, but fortunately the broken 5th gear of CLK-GTR in NFSIII has been fixed.

CLK-GTR vs F1 - Some tracks are slightly faster with CLK-GTR, but most tracks are faster with F1.

Shifting - Shifting in NFSIV is a bit faster.

Curbs - Curbs in NFSIV have much more impact on your car's speed.

Perfect landing - As described in my tips and tricks guide, perfect landing is way more rare than in NFSIII, about 10x more, so you really can't grind tracks using this trick.

Scaling - Cars in NFSIV are much bigger, therefore all times on NFSIII tracks are much faster. In NFSIII, cars sometimes look like RC car, especially with far cameras.

Weather - Weather in NFSIV have much more impact on lap times, usually about 3-4 seconds per one minute. In NFSIII, it's only about 1 second.

Surface - Surfaces like snow or dirt have less impact on car's handling in NFSIV, so you can drive over snow just fine.

Automatic downshift - If you slow down too much, usually because you crashed, NFSIV will automatically downshift to 3rd gear. The car can also sometimes enter a weird state when it suddenly steers way more than it should. I can't explain it.

Tournament/knockouts patterns - NFSIV always have fixed patterns, including career, which makes them fun to run.

Bonus Porsche - The first version of NFSIV have Bonus Porsche as AAA class car. That was later changed to AA class.

Jumps - In NFSIV, it's faster to let go of accelerate during jumps and also gear matters upon landing. In NFSIII, it doesn't matter.

Opponent cars - AI in NFSIV is much more aggressive and if you just touch them, they will immediately try to crash into you.

Shortcuts - Some shortcuts on NFSIII tracks are much harder, like forest in CW or town chicane in SU, due to cars being bigger in NFSIV.