I would imagine a 4 stroke would be less likely to dump you if you give it too much throttle. And mixing fuel is an inconvenience. But it sounds like a 250 four stroke is not enough, and I've read a bunch of posts saying 4 strokes are unreliable and require more maintenance. I don't see why, pretty much everything is 4 stroke these days and it's not difficult to adjust valve clearance and such, but that's what I've read.
Basically I've read a lot of mixed opinions. It sounds like a 450 four stroke is the way to go, trick is finding one used for a good price.