There is no such thing as a 'ripe' onion so you never have to wait for that.

The usual advice is to wait for the tops to brown and fall over. The trouble with this advice is that sometimes the tops don't actually do this until well after bulb growth is done.

The logic behind this advice is the presumption that you want the largest bulb the plant can produce and therefore leave them in the ground until the tops are dead and the bulb clearly can't grow any larger.

Forget about that. Harvest onions when you want to eat them. If your bulbs are 4" in diameter I guess the question is why wait any longer? They are already good sized onions so what do you have to gain by waiting?

Don't know about z5 but here onions will start to divide at the bottoms if left too long.