So think of THPS4 as a nice improvement to THPS3, by taking the 2min runs out, adding pro challenges etc.
Essentially, it improves by axing the two minute run, and just letting you cruise through levels and find objectives, which is also nice because when using the Xbox custom soundtracks you can actually just listen to entire songs (or CD's) when playing the game, rather then just the first 2min of everything, its definitely a nice change.
Next up, the game also finally does not require you to beat the game with every single person, its very nice to see that when I beat objectives with Rodney Mullen it carries them over to all of the skateboarders.
Another nice welcome feature is the new Pro Challenges (which are the real reason to play as the different skateboarders). Basically you'll do challenges specific to the real skateboarders personality/lifestyle, which makes them a lot more interesting, especially if your a fan of any of the particular riders.
So think of THPS4 as a nice improvement to THPS3, by taking the 2min runs out, adding pro challenges, not forcing you to repeat objectives, and finally bringing everyone's favorite Vert dog (Bob Burnquist) back.