If all three full backs are on two year deals I think we have enough, with Richman as emergency cover. Personally full back is the one area I'd have offered one year deals to see how they get on against the wingers in the higher league, but if PP is confident enough in all three of them to offer them two year deals then there is no reason to get a fourth fb in because one of the existing three wouldn't even make the bench for two years.
Similarly seems odd to give Williams and Richman two year deals when they couldn't get in the first team in the north, but again if PP is happy with them as squad players then we have to trust him. We just need that Lundstram shaped hole in our midfield filled now.