I had this issue and I made a change that fixed it for me - it has to do with the magazine that is loaded in your M4 when you first pick it up.
The mag that's in the M4 when you first pick it up is not actually the same as the ones that are in our Arsenal box. My fix was to pick up an M4 out of the Arsenal, remove the magazine that's in it, drop it, then save the loadout.
Or I guess you could remove that mag, drop it, pick up a fresh mag from the Arsenal and load that, then save. This way you wouldn't load every loadout Condition 4. Note that this is not how my (now working) loadouts are saved so I can't guarantee it works, but I can't see why it wouldn't.
I encountered the same not-loading issue with my SAW loadout despite the fact that there's no M4, but I think it was because I was starting from a "base loadout" that used an M4 then changing to the SAW. Building it from scratch after clearing your loadout *should* sidestep this issue but ARMA being ARMA who knows?