I think what Roxy is saying is that the marlin codes does not allow for different number of points in x vs y right now. Being code, it can of course be changed, but that would require a patch and a reason to accept the patch. That's somebody's work both now and for future maintenance and would need to be justified.

Right now your 8"X12" bed with a 4x4 grid will have 4 points in X separated by ~2-2.5" and 4 points in Y separated by ~3.5-4". Is that a problem? Is there a strong use case for e.g. 3x4 probe instead of 4x4? If so, maybe someone will help write the patch and argue for inclusion.