I chose four because it would cover most bases and/or scenarios to include hunting, large game/small game and ccw/edc and/or self-defense. Self defense could be done with just a .22 as well as most hunting, but for a well-rounded arsenal four allows some overlap where specific firearms would be better than others.
The shotgun would be best for birds, rabbit, squirrels and even deer, hogs, bears, whatever game you can get reasonably close to with the right ammo, but not best for edc unless your in the wilds 24/7.
The handgun would be best for ccw edc, but not so good for hunting unless you can get up close.
The .22 is good for small game and even can take larger game if close enough and is usually light enough to carry around but not very concealable and could be used for self-defense if necessary.
The 6.8 AR would be good for defense and hunting larger game, not so much for smaller game and birds, not a edc unless your outdoors in the wilds all the time or its permitted in populated areas, which is mostly not likely unless you live in Israel.
But if I had to choose only three I'd probably lose the pistol and keep the AR, if only two I'd probably lose the AR and keep the shotty with an 18" bbl and the .22 Marlin. If it came down to just one, I'd have to keep the 870, it's just too versatile to not have.