I started with the 4-blades from DSC, which were a bit average (went from a Gillette Fusion).
Changed to the 6 blade and they are a lot better, but I would say the Fusion is still a better shave.
I cannot stand the prices of the Fusion refills, so will stick with the DSC 6-blades for now (I have a heap of them as I get 4 blades a month, and probably use 3 every two months - must go and change my plan to bi-monthly).
The 6-blade refills do not fit on the 4-blade handles (and vice-versa). DSC send you a new handle when you change your subscription.