You can have either, if you have 4 it's an ff in the fibre stakes or if you have 2 it's helps with something else! Sorry I'm in bed and books are in the other room. In a nutshell you can do either ands a hex!
No they aren't free they are a hex b, it's just that if you only have two of them they speed your weight loss rather than having 4 which gives you plenty of fibre!