PI mechanics question, 4-man crew

[Welpe]

Everywhere I've worked, it's always been taboo for the Umpire to throw a PI flag. We'll turn with the pass to help rule on incomplets on trapped balls and if a ball was touched but should not be throwing PI.

As you said, there are some serious coverage gaps with a four man crew and there could be extenuating circumstances where the umpire might throw on PI but in your case, it doesn't sound like that it fits.