handling does not cause regurge in ball pythons.
Regurge is a flight or fight thing, they do it to get away from what they see as a potential predator.
The snake is stressed out badly if it regurges.
When in the wild a snake goes out and stalks a prey item, and gets it eaten, the traveling back to the den if it didn't find the rodent in a den, would be much more bumpy than the little bit of handling you might do.

4 days after a meal it should have been digested by then, so I ask, did the snake have a hot spot to digest on?
Did it sit on the cold side due to stress?
Lots of things come to mind when a snake regurges, but handling them isn't one of them imo..