If you don't control a creature with power 4 or greater as your combat phase begins, Nessian Hornbeetle's ability doesn't trigger. If you don't control one as the ability resolves, it has no effect. It doesn't have to be the same creature at both times, however.

Nessian Hornbeetle's ability only gives it one +1/+1 counter, no matter how many creatures with power 4 or greater you control beyond the first.