So - if the rules stated “If you didn’t plunge it, don’t play it” - this would be a very simple case. The problem here is that player 4 is allowed to try to trap up and gain control of their ball if it gets autoplunged and declare that they are continuing. There are a lot of reasons to have the try to gain control rule (we won’t recreate state being the main one, so if player 4 has done setup for ball 3 and this situation arises - losing their ball will be disasterous), but it can lead to something like this. Heck - there are games where it won’t be easy to tell who’s turn it is quickly because there is something overriding the score display.
I imagine this scenario like this - ACDC - i’ve got all 3 multiballs lit and I’m down significantly and as I am stepping up a ball is leaving the pops. It’s going to bounce over from left to right for an easy hold up the flipper trap. The ball save isn’t blinking. I’m down 40m. Are you really saying I’m stuck between pressing in the button and possibly get DQ’d (it’s not my ball) or letting it go and start from scratch (it’s my ball)?
Actually in trying to address this question I understand the ruling completely now. The ruling is an extension of “That’s Pinball” and it makes perfect sense.