Are all AEG M4 grips interchangeable?

I use grip spacers. Mancraft makes some but I just cut my own with my mill.

~95% are compatible. Some of a step spacer inside the grip (Marui 416)...

For the most part, yeah. You may come across some fitment issues, but those can often be fixed with shims or sanding.
Some grips are worse because they put your motor at a weird angle because of how they're shaped/angled.

As with most things in this sport, it depends. If your receiver is newer and relatively close to tm spec you'll likely be able to make most grips fit without issue.

I mostly work on older guns and it's hit or miss with those. My primary is a ~2008 UTG and it is so close to TM spec its amazing: take your pick, almost anything will fit. My madbull or old Bravo receivers though, haaa good luck. PTS, KA, CA, Lonex, cyma, stryke, etc etc - nothing fits right except the factory A2

It's been hit or miss for me, especially fitting old grips to newer receivers and gearboxes. I actually have a build right now that needs more sanding on the grip, because the shimming sounds bad (when it shouldn't), and I suspect it's because of the grip needing more sanding to have the proper angle.

For reference, I'm fitting an old Classic Army grip Guges sent me to a Matrix receiver and a Lonex gearbox shell.