Type 4 is a "divide by zero" error. It's a problem in computer code since obviously, if you divide anything by zero, the answer is zero. That makes an invalid answer and so it's generally regarded as an illegal instruction.
In practice, what you're describing would probably indicate a flawed or corrupted line of code in the app. This theory is reinforced by your assertion that a backup copy of the app runs fine. So, the question becomes, why don't you just replace the app with the backup and go on? That is, after all, why you have a backup, isn't it?
Or, is there some other thing going on that you've left out?