My 4 year old did that when he was 3 1/2 . I'm not sure why. We did have him checked one time for a UTI but he didn't have that. (He had a fever so we took him in, but that turned out to be strep throat, go figure). He would pee and then go outside for 3 minutes and then wanna pee again. This didn't happen all the time, but almost when he was excited and running around a lot. (I remember going to the bathroom as a child, running outside to play hide and seek, and as soon as I hid, thinking I needed to go to the bathroom even though I'd just been....not sure why or if it was the same kind of thing). We asked him if it hurt, itched, burned, whatever. (He always said no). We thought it may be an attention thing (we had a new baby too) or that he was wanting rewards for peeing on the potty. But that didn't seem to be it, because he would do the same thing if he was at a playdate with his friend, when I wasn't even present. We actually started thinking it might just be him being paranoid about peeing in his underwear, and wanting to run to the toilet anytime he had the slightest sensation to avoid an accident. He hasn't done that in a long time now though. Actually forgot about it til I saw your question.