The MMC2 and MMC4 function almost EXACTLY the same as a whole. The only known differences (so far) are as follows:
MMC2 - 40-pin shrink-DIP; 8KB swappable bank at $8000-$9FFF, last 24KB hardwired at $A000-$FFFF
MMC4 - 44-pin TQFP; 16KB swappable bank at $8000-$BFFF, last 16KB hardwired at $C000-$FFFF; support for 8KB SRAM at $6000-$7FFF, always enabled.
Both chips have the same CHR bankswitching setup (with latches in BOTH banks).
P.S. If you don't get this note, let me know and I'll write you another.