Can I use the M1 Mac mini for an iMac G4 retrofit?

What do you mean by "retrofit"? There are no parts you can use from one in the other. You're talking multiple generations of hardware difference. You might as well put the entire mini inside the G4 box and run cables into through the empty port slots. You certainly won't be able to run G4 software. I use my old G4 (with all contents removed) to store spare printer cartridges.

Duder967 wrote:

I mean a project where i put the guts of the m4 into the g4. thought it would be a good mix of classic vs modern. what are your thoughts

Such a project is well beyond the scope of this forum.

It is very likely that there would be cooling considerations if you put the M4 Mac mini inside the iMac G4 chassis, and if you're talking about the sort of work required to modify the iMac G4 and turn it into an external display, my impression is that (a) it would be very extensive, and (b) Apple would not want us to talk about it here.

Duder967 wrote:

I mean a project where i put the guts of the m4 into the g4.

I look at my mini and I look at my G4 and I could fit the entire computer into the G4 case, plus room left over. If you mean replace the CPU with the CPU and the drive with the drive, there's no point in doing it that way. Most things are soldered together these days, and memory is integrated. There is no classic RAM and video RAM. The parts you want to replace individually may not exist as individual components anymore, and even if they do, they are probably all soldered together. You probably can't even replace parts in mini models that are only one year apart with those from a different year.

As I said earlier, if you want to use the case of a G4 with the workings of a mini, just put the entire mini inside the G4 case and replace the outside ports with ones connected by cables to the mini inside. That would be relatively easy and cheap. The one other thing you might want to do is make sure you have a venting fan so it doesn't overheat.