It has to clear the ground
There really is no minimum but I have read about ground noise creeping in on low dishes. 6' would treat you much better than a 4' for
C Band. It is so funny associating 4 & 6 with big dish
The BSC lnbf's would work best for your application IMHO. You could even get rid of the smaller dish if you desired since the BSC units do both C and Ku. The larger dish would make Ku much better.