Umm... I have to ask... but are you housing all four of those snakes in the same enclosure?
If yes, the reason for the regurge is obvious. That snake is probably being stressed out by his bigger roommates and he can't find a suitable place to sit down and digest his meal, so he vomits.
If not, then there could have been a number of factors: the meal was too big, or it wasn't warm enough, or there wasn't a good hiding place, was too dry, he's got parasites, etc etc. could be anything.
-Lemur 6