Once the roos begin to mature those banty's won't know they are half the size of a standard roo. Eventually you will have fighting if all 4 live in the same pen. Even more so if there are females in there. Mating males can be vicious to other males. You may end up with blood shed and death. If you want to keep 4 males you might want to think about 4 pens or 3 pens and 1 in with your hens.
I find my banties have ego problems and think they can take the hens from the standard roo... being 1/5 of the big roos size puts them on the losing end EVERY time. I have 3 roos and each has their own coop but share as much free range area as they want. For the most part, they avoid each other and keep on their own turf. However, I would not keep them in the same pen as someone would get hurt bad, and my guess it will be the banties.