It all depends on the specific variety and still varies from season to season- depending mostly on when the trees flower. Z4 is not a place where all plums can survive and I'm not experienced with the ones that do, but plums should be somewhat soft before they are ready to pick. If they are then juicy and quite sweet they are ripe. They also pull from the stem quite easily.

I just harvested my first european plum, Opal, here in zone 5a.

My Japanese plum, Shiro, is not fully ripe yet. This is supposed to be an early J. plum.

Hi, My Italian prune plum does not ripen until the very end of Sept. Mrs. G

I also have an Italian prune plum that ripens late Sept/early Oct. And I have 2 Yakima plums, a hardy European type plum. It looks a lot like the Italian, but bigger and the skin is a bit redder. Makes a beautiful, tasty jam.
Just keep picking a plum every now and then and sampling. We pretty much need a frost before the Italian plums get real sweet. Then the race is on to beat the deer to the plums!