Wow, I found the cause of a "random" crash in Modern Combat 4: Zero Hour.
When FMJ rounds or Screecher rounds equipped and firing through an enemy and hitting an other enemy.
So basically the only thing FMJ rounds are designed for in MC4 causes a crash.
Tested today, three times in a row it happened when two enemies standing behind each other, and firing at the front one wih FMJ rounds in a VECT9. iPad Mini 2, iOS 7.1.1
EDIT: today it also crashed with this situation but with Screechers (SOCAR-S). I suspect that is was because of the same bug.
I'm very happy I found this, because all I have to do now is using different bullets. I can play MC4 again!
(Now we're at it: playing MC3 and MC4 after playing MC5 feels so awesome. Just every single little thing in those games is better. The maps, the mechanics, the weapon balance (except the Compakt in MC4), the controls, the animations, the UI design, the sounds, the textures.... Everything.)