If one or more +1/+1 counters would be put on a creature you control, that many plus one +1/+1 counters are put on it instead.
It’s a replacement effect, adding one more counter to the existing effect, not creating a new effect that adds an additional counter. Keywords here are “if” and “instead”.
It’s a replacement effect, adding one more counter to the existing effect, not creating a new effect that adds an additional counter. Keywords here are “if” and “instead”.
TL;DR - 3 Counters, not 4