From a quick look at the picture, either should work. The M4 would probably be more efficient.
The case you have converts AC to DC and feeds it in. Inside, the circuit boards convert the DC levels to 3.3, 5, etc and then send it out via ATX connectors and SATA connectors and whatever else. If you feed it the voltage it requires (with the Carnetix) then its a simple swap.
If you add an M4, you would take all those electronics out. You shouldn't lose any functionality though... the M4 would just replace all that and you'd feed it a 12v instead.
Whatever is easier / cheaper I guess...