4M has smaller flash memory, whereas 8M is just that. 8M supports current firmware, where 4M is cut off at R09.xx.xx/DSP R16.xx.xx.
The RAM22 processor has some DSP issues, whereas the BRAVO122 process seems to have a lot less problems. As of R14.xx.xx DSP firmware, the RAM22 processor started acting up with button presses (lagging/beeping). Some members here are quite familiar with RAM22 I'm sure. Motorola refuses to acknowledge the problem.