In my opinion, you've got play this one down and hope for the best.
Question one: Would anyone have entered preflop with one 4? Possible, but unlikely (but then again, it is microstakes).
Question two: Where is he seated? If someone is playing a 4, they are either in the blind or sitting on a pair of them. Since the latter is impossible, the only way I seriously put him on a 4 is if he is in the blind and lucked out. If he is in late position, I take this as a stone cold positional bluff.
One thing that is always good to do in limit play (with a great hand and trouble on the board) is raise and wait for the reaction. Especially if you are unsure of what your opponent has and think he may be bluffing. By calling, you are keeping him in the hand and aren't gathering any info. By raising, you are taking the role of aggressor and forcing him to react to you. If he calls, I know I am probably in the lead. If he reraises, I'd be willing to dump...but at least I got the info I was looking for.