It simply does not spawn. There seems to be an issue with its handguard not existing, causing the weapon to not spawn. All currently existing m4s are duped versions of one or two m4s that were found some hours after 57 release that has been duped hundreds of times like a wildfire. A minipatch shortly after removed the m4 entirely from the loot table. That is the theory, because all m4s being used are identical atm. There are also no custom buttstocks and only the MP handguard or no handguard at all.

Apparently the devs are not away of this issue and think the m4 actually spawns because a lot of duped ones are in circulation.
Please investigate amd fix this for the upcoming stable patch, a full stable release without m4s actually spawning would be pretty sad!