Bonding a fourth - male or female?

I currently have a trio that consists of two females and a male (all neutered). Whilst one of the females is very close to the male the other female is often left on her own. We are hoping that adding a fourth might change the dynamics (or at least give the lone female more company). If we were to do so would it better to have a neutered male or a neutered female? We understand it might not work at all!

Thanks