My M4 arrived this morning, and I have to say (even with the issue I'm about to mention), it's an awesome piece of kit!
My issue is as follows: Using the firmware that was installed when I got it (2.0.5b1), no matter what I do to set the LowerROM to be the modified 6128 ROM, it fails miserably to boot properly (if at all), but downgraded to 2.0.4 and it works as expected. Tested with Stunt Car Racer (before: no matter what, it would ask for a disk in A:, after: with the LowerROM, it runs properly, but I haven't tested without!)
I have a feeling some of the issues are with the age of my machine and possible oxidation on the edge connector (I've cleaned it, but it didn't work at all the first couple of times I plugged it in, so it may need a little more TLC) but downgrading seemed to fix "all" my issues (well, those I've tested so far).
I also need to say a BIG thank you to Duke for his patience during my order.