The problem is there is a voltage drop on the MotherX4 board, dropping the voltage in some cases below what the AMS1117-3.3V regulator on the M4 needs to operate correctly.
That being said it may operate a bit out of spec, some have reported no issues powering it directly from the MX4 board, for me it works with MX4 when using external PSU connected to MX4. I can't remember my voltage readings, but it was a little higher with the external PSU.
In the next batch of M4's (whenever I receive the pcbs...) will probably be fitted with an AP2114-3.3V regulator instead, that should solve the issue.
If it works as expected, one could replace the AMS1117 and two caps. Or better solution use my adapter board (if one has that) and plug MX4 after M4.
@jomicamp what CPC model you plan to use for it, then I can better advise on the configuration.