I know there is a 4x slotexpander, but not sure how I configure it. not sure how to get 8 (And I would assume 4 subslots was max per slot)?
I know exta and extb is slot 1 and 2, but how to get -ext into those 8 subslots?
Now I have some more time to clarify... in openMSX, I/O only extensions do not take up any slot (V9990, OPL4 and some more). (This is also how an 8-slot expander on real MSX works: some slots are only usable for I/O only devices.) You can see this with the OSD menu, it shows you which slots there are and which are used and which not. I/O only extensions like V9990 and OPL4 will show as not occupying any slot.
You can also explicitly put a certain extension or ROM in a certain slot. If you first insert the slotexpander in a primary slot, you'll see the extra subslots appear, including an indication of the actual slot number it corresponds to.
Please try it out. This way you can control the slots exactly as you would on a real MSX.
PS: I realize this is probably not relevant anymore, but for a next time you know now 
