Yes, that’s it. The M4 doesn’t come in a 7.62 - you would have to go with a Springfield Armory M1-A family - SOCOM or Scout rifle. (none of this means anything to me - just repeating what he says.)
I’ve heard there are upper receivers for M4s chambered for 7.62. I haven’t done much in the way of research. I know that H&K makes a rifle called the M417 (the 5.56 version is the M416) that is chambered for 7.62.