first of all a 4x8 does not support the bandwith needed for the ka/ku,second of all you need to check what bandwith your dual powerpass diplexers support. you do need to use 2 6x8 switches that support the ka/ku and the only reason to stack switches is if you are using more than 8 klines out either 8 regular receivers or 6 regular and 1 dvr so on and so forth let us know how many receivers you are using so we can further help you out.
This is CABL.com posting #166784. Tiny Link: cabl.co/mRye