I think a good baseline assumption is that most ppl 3b and either 5b or fold to a 4b (ie: don't call often).

I think under that common assumption this 4b pre is fine. It's good for blockers (blocking his AQ/AK/AA/99) that might 3b/5b, and obviously A9o isn't good enough to call OOP here.

Given that we now know he flats 4b, we might want to start 4b bluff more playable hands like KQ/KJ/AJ etc.