The CompM4 is described in the game as:

Offers 2x zoom

It can be found in south-west Ocoro province, in a Santa Blanca outpost next to Campo Ana Town. The accessory case is in the armoury on the north side of the base, near the power generator.

I approached the outpost from the east, and there’s some cartel in a tower and a checkpoint near some train lines. After crossing the rail track, you can sneak around the north side of the base and take down the occasional sicario which are spaced out, and turn off the power (just in case) as you approach the armoury.

The CompM4 scope is for SMGs and ARs and has two reticles which offer slightly differing zoom levels, a regular red dot (1.0x I assume, like all other red dots) and a 2.0x zoomed reticle.

The two zoom levels are too similar to be much use for me, so I’d look to some of the alternative scopes if you want added accuracy for close engagements.