Non of 3.7 hack isn't gonna work on vb4 uless it's not updated. For more info check vBulletin 4 mods/hacks section and see if those hacks are listed there. If yes you should be good. No then don't even think about it.
Good Luck ..
This is not true. Some plugins for 3.5.x work with 4.0.
This is why I say 'existing ones can cause errors'.