The 2 MX4 slots are more than sufficient given that with the RSF3 you don't need to have as many as a Mother X4 given that it already includes:
*a 2 MB memory extension (can be deactivated if necessary): no need to add another one.
*255 rombanks: no need to add 1 a rombank.
*M4 Board emulation: no need to add the card of the same name.
*Hacker v7.0 in a dedicated rombank: no need to add the card of the same name.