Frog poop looks like.... well, frog poop. You'l know it when you see it, these guys poop big for their size. A brown weenie is what your looking for.
Could you post a picture of your frog? Don't give up with tongs, keep coaxing him, and eventually he will submit. It's not hard, just gently keep running his lip from side to side and you will invoke a bite response. If he turns away, follow him. Just make sure to not stress him out, I do this for a maximum of 15 minutes tops.

4 days should get you a poop, I'd say, next time after feeding, drop him in his waterdish and feel his right side, if it's not hard, you shouldn't worry. Also, while doing this, dig around in his burrow, he might have hidden it somewhere!