What would the 9-team finals format be?
You could keep the top 4, but my preference would be top 6:
1 and 2 go to semis
3-6 and 4-5 single game elimination, with winners advancing to semis
Semis as per normal
6/9 is probably too much from a percentage point of view but at the same time 4/9 is probably too few. 50% is the magic number but you can't get that with 9 so which is it? 6 would work with 10 and 11 teams too so from a consistency stand point may be good.
As a reference, in NBA greater than 50% make playoffs but in AFL less than 50% make it.