Ah you are right, the functionality from scenario are colliding a bit with the standardmap here
To achive what you want you must currently use the "scenario" playersetup. Then you can specify exactly what colors/teams you want for the map.
If you take a pre-set teamsetup, like 2:2:2 or 3:3 for a standard map you then decide which startingpositions (colors) should be available per team, but since this is a scenariomap you will get exactly the same color/startposition as you choose as is specified in the mapeditor. (makes sense?)
Im not sure how to make a good solution for this, as you have the option "Random team" in the dropdown also - how would this be represented in the gamelobby? Its a bit tricky. Maybe the better solution would be to remove the "Random team" option from the dropdown of all teamsetups for scenariomaps, so that you must specify exact teams, then I think they could be better represented in the gamelobby when hosting scenariomaps.
Ill think a bit about this
But for now, please use the "scenario" playersetup if you like to have a specific 2v2v2v2 scenariogame