If you already had got 5 ribbons in multiplayer before the September Update is installed, this will unlock retroactively once it has been installed and you have booted up Gears 4 again.
If you haven't met the requirements yet, these are easy ribbons to get (Can be done in casual cop-op vs A.I):
Igor - Got 10 assists in a single Versus round (simply do damage to an enemy, then let someone else kill them)
Methodical - Executed 5 enemies (Knock enemy down and curb stomp them, rinse and repeat 5 times)
Pistoleer - Had the most pistol kills in Versus or Horde (Just kill a few enemies with a pistol, most people don't use pistols so shouldn't take many)
Quick Clips - Had the most perfect active reloads in Versus or Horde (Shoot off a few rounds, active reload, rinse and repeat half a dozen times, then play as normal)
Downer - Downed 5 enemies who were then killed by another player (Self explanatory, just make sure there's a team mate close by)