Moreover Goku as a child is much weaker than Adult Goku, don't know how much exactly though I'm not sure if Goku as child is any stronger than DBZ Goku.
2. SSJ x50, SSJ2 x100, SSJ3 x400... i don't know if there ever was a official multiplier for SSJ4.But would be interesting to know if SSJ4 is Child Goku x (lets say) 4000, or if its Adult Goku (because the magic effect is broken by the transformation) x4000 which would end up in a powerincrease of x8000 or something like that.
3. Zenkai isn't much a factor post-freezer Saga anymore. Goku may have gotten stronger throughout DBGT, but Vegeta still wasn't even able to go SSJ3 so he most likely didn't become much stronger outside of SSJ4.
I still think Super Buu (/w Gohan) was most likely as strong as some DBGT characters and even Baseform Vegito was stronger than him. So if Super Buu was about as strong as Baby, then SSJ Vegito (who was infinitley stronger than Super Buu) must be at least as strong as SSJ4 Goku when he is killing Baby.