Well first of all this is a worthless debate because Monica would never be on a tour that Brandy is on

And four acts is a lot especially because they all need and deserve equal amount of time. We're talking 4 hours including intermissions and setting up. Not realistic.
If we had four acts one would have to be a legit opening act and okay with doing 20 mins and Monica and Jazzmine are too established and have too much material for that to work for them.
I can see three performers: Jazzmine, Brandy and Fannie in that order. I love Brandy going in the middle and Fantasia closing it out.