I actually think they could. Based on what I've seen in GT, goku could barely take on baby vegeta as a SSJ3, but kicked his ass in his final form as a SSJ4. I think it would be a similar case against Super Buu.
not against buuhan, mystic gohan>>>Super Buu>>>>>>>>Ssj Gotenks>=Ssj3 goku So Super Buu+Mystic Gohan+Piccolo+Goten+Trunks=Would be a great difficulty,he can win,but it won't b easy,i see it like this:[link]
Yes, I know that and I saw that video a long time ago. That's what I'm saying, SSJ4 Goku & SSJ4 Vegeta could beat Buuhan together without fusing, I didn't say it would be easy. Anyway, that's just my opinion.