We kind of have this problem. My 9 year old loved things when he was 5, but he was scared last June. We finally got him to do all 4 "mountains" (mine train, and the 3 others), but he would only do them once and he was terrified.
My 2 year olds, at the time, didn't have a problem with splitting up to do these things because they are so used to the idea that big brother is older, due to the large age gap.
So then we did adult only in Feb and fulfilled our need for thrill rides
On our upcoming June trip, I didn't even get FP for any of the mountains. DH and I had our fill in Feb, and I'm not going to go through splitting up if older DS is against it. It is just a pain.
Our only dilemma is 7DMT. My daughter meets the height requirement, her twin brother is iffy (with shoes, he's right on the cusp, so it will depend how tall he stands, the CM, etc.) However, my younger DS is much more adventurous than his twin sister or older brother. So if my younger DS doesn't meet the height, I totally see DH or I having to ride Mine Train with children who won't enjoy it.
In other words, my best advice is to split up for a few rides that you think older kid will enjoy (Soarin' for example isn't as scary...also the Land is right next door and my 2 year old twins just adored that ride or you can get a fun snack at Sunshine Seasons). Maybe try for FP+ for 7DMT because those are so hard to get, but otherwise, don't plan on separating much...and with FP+...make the FP+ for one adult and the older kid at the height restricted ride and make another for one adult and the 2 year old at a nearby not restricted ride (or choose a ride nearby that won't have a wait).
Other than Soarin', (maybe Test Track), and 7DMT...you will probably easily be able to switch FP+ if you get to the park and all of the sudden 4 year old just HAS to ride Big Thunder Mountain or something.
But, really, there is enough to do, at least for my family, that has no height restriction that we all have a good time WITHOUT splitting up...ages 30s, 9-10, and 2-3
ETA:
Some ideas for things to do with 2 year old
Soarin' has the Land and Sunshine Seasons nearby
Mission: SPACE has a cool indoor play space
Test Track empties into a cool car display thing
Splash Moutain has a neat playground nearby
DINOSAUR (which is really scary, IMHO) has the dinosaurs to play on nearby
I'm sure other people will have others, but those rides you wouldn't necessarily need another simultaneous FP+