I think "Seeker13" is referring to the 4 slots available for your 4 active gang members.
So rather than just having 4 of the six Morningstar members active in your crew at one time, you can have all 6 members active at the same time.
Or you could have 1 Morningstar, 1 Deckers, 1 Luchadores, 1 Mascot, 1 Biker, and 1 Cop all active at the same time. Giving the gang more variety when spawning on the streets of Steelport.
I can't tell you how many times I've passed a group of 3 or 4 Saints, all of whom are the exact same model. So having 6 active gang members instead of 4 might lower the chances of seeing triplets everywhere.
Of course I could be wrong, but it sounds like that's what they were referring to from my point of view