Hi and welcome to RO.
If you Cascade the MP4 into the MG24/14FX (you would come out of the unpowered "Main" jack on the FRONT of the MP4 into an input on the MG24. In which case you're only gaining 3 channels, because you sacrifice a channel from the MG24. If you accidentally plug into the powered 50w "Speaker Jack" on the back - that could get ugly and damage something on both mixers before long. Seems kinda risky to add 3 channels.
Others may have more ideas, but I'd much rather cascade the unpowered 12-channel Yamaha mixer into the MG24/14FX and use the MG24 to control anything that has to be present in the monitors. Since you're using a fairly modest amplifier, I'm going to stick my neck out and assume you're not a super-loud band. So it would follow you're not pumping drums (if you have any) back into monitors as some bands might on a big stage with a loud mix. So in your case for instance, you might submix a drum kit to the 12-channel mixer -> cascade the output(s) to 1 (or 2) inputs on the MG24. Leaving the rest of the channels on the MG for vocals, keys, bass, guits, and fiddles.
You may not use drums, I don't know.... So not knowing what instruments you're using, it's hard to be any more specific.
If you want to post some follow-up info - it might open up some other ideas.
Hopefully that nudges you in the right direction. Good luck!