I agree with lvoss… most times in a home situation, "guest" are not welcome unless they have know each other as pups... example... Fatia loved when her brother Tego would visit... others, not so much and particularly other bitches... Loose on neutual territory she was fine as were most of mine. Many times at lure trials those of us that stayed over in RV's would turn the "hounds" loose in the evening for a fun romp....
I too find it a bit odd that none of the four can run together... unless they were all the same sex and have had unpleasant interactions either when in season or if males, around bitches in season...
As a breeder I don't try and socialize mine to accept strange B's in our home, if they are OK with it, fine.. if not..well, it is their house... as that is their territory.... All that said usually Opposite sexes will be ok... but not same sex... related or not... remember unless they see each other all the time.. they really do not remember