Originally Posted by KindaShy

Yeah, I believe 4* and above slightly increases the group ability whereas a 3* actually adds one (since they did not have one previously).

Still looking for my 1st 4* from normals!!!

You'd be wrong then. While it seems the trend to increase the ability they have they can gain an additional ability instead of enhancing one. I don't have a wide pool to make a generalization, but Toad Lily gains an attack boosting battle ability in addition to her default 50% reduction from pests ability upon evolution.

If I had to make a generalization 3* adds an ability since they lack them.
4* improves the ability they have.
5* Gains a different ability in addition to what they have.
6* Gains an additional ability and maybe improves what they have as well to fit the trend.