We don't get/take many opportunities to go coaster-ing, so my kids aren't used to them. My wife convinced my 8yo daughter to join her and 14yo daughter on Jurassic Park. They take the pic at the top of the 84ft drop at the end, and her eyes are literally (<---------THAT--------->) big.
She survived it nonetheless, and then we all went on the Mummy (9yo daughter didn't want to ride JP, but we talked her into Mummy). My 9yo probably would have been better off with JP, as they were both glued to their seats on Mummy. We had a hard time getting them on anything else the rest of the day. lol (thawed 'em out at DL and CA the next couple of days tho)
That said, those two rides alone are probably too much for a 4yo, unless he's been riding coasters since he left the womb. Backdraft was interesting (in my opinion), but my girls acted scared (not sure if it was from the heat or what). Not to mention, you have to go down 8-10 escalators to get to this area of the park.
Back to the Future was a bit intense, tho it's just like Star Tours. Your son would probably enjoy Shrek 4D, tho like the movies there are some not-quite-for-yunguns jokes. As mentioned, there's the Nickelodeon area (play with some foam balls, and/or get soaked...there's a jungle gym or somesuch in back that we missed).
Animal Planet Live was great, my 9yo has been acting like a monkey for a week since.
lol I felt the Studio Tour was boring, at least at the beginning (yeah sure, it's great that some show was filmed in that non-descript building, but put something on display to represent it!!), but it did have its high points. Didn't get around to Van Helsing.
If your son is a theme park veteran (as veteran as a 4yo can be, lol), then USH would probably be ok. However, I'd have to say that it's more geared towards the pre-teen/teen set. Otherwise, the Nickelodeon play area and meeting SpongeBob and Dora are about it.
On the whole, it seemed to me that USH felt "old". It had little ambiance, like you get at DL. It felt more like it was a collection of things to do, rather than things to
experience.