It sounds to me like he may have had a bm in his pullup that went undetected and unchanged for awhile. Did he have a recent bout with diarrhea? That will make for a painful diaper rash. Also, at four, his skin probably is not used to having poop next to it and it will be more sensitive than it would if he were younger and still in diapers all the time. And, a four year olds diet is so varied that many things could cause the poop to be a little rougher on the hiney. Even a lot of citrus in the diet could make the poop more likely to cause a rash.
Unless he has been on antibiotics recently, I would doubt it was yeast in a four year old boy. Even then, I would not think of that first. I would say that he needs to soak in a bath every night until it is gone and have some diaper rash cream put on his hiney every night before that pull up goes on to protect his skin.