Forgot to say why it's a fold preflop. You don'thave enough implied odds. You have $4.50 in your stack and you have to call $0.50c to see the flop. That's only ~9:1 implied odds. I believe you make a set on the flop either 8:1 or 9:1. It would seem as though you are getting enough odds, however, that is assuming you get villians entire stack, which isn't always the case. Since you don't always get his whole stack you need to be looking for something like 15:1 or 20:1 before calling preflop with a small-mid pp is a +ev play.