If the dude going all in is a decent player, then I would most likely insta fold.
Early pos limper - calls your 4xBB raise and the pushes on pretty harmless flop. You will have to think that he puts you on a pretty decent holding as well after your pre flop action, but still pushes. What kind of hands would you have done that with in his/her's shoes?
You should be "very careful" playing for stacks with only a pair - so if you dont have very good reason to call - fold.
But then again, i suck at cash