Why do M4 guns appear in Stranger Things?

Inconclusive

The pictures you've provided are dreadfully unclear, so it's impossible to tell if we're actually looking at an M4 carbine as claimed.

The "Internet Movie Firearms Database" believes they are Colt Model 723, which is a variation of the M16A2, the same weapon the M4 Carbine is based on. The M16A2 was officially adopted by US Department of Defense in 1982, for use by the US Marine Corps.

Without clearer imagery, I think it's difficult to answer precisely which weapon was actually used. It's possible (but again inconclusive) that they are M4s - it may have been what the prop department had on hand, and because it is not a military or historical piece, they used them because they were "close enough". Unless they're going for absolutely accuracy, Hollywood prop departments tend to use what they have.

_{An example aside, consider Timecop and Stargate: Atlantis. Two weapons modified for the film, one of them very distinctive, show up on an entirely unrelated television series. It's highly likely somebody in the prop department was asked for a "futuristic" gun and said "How 'bout this one we already have?"}