So, I strongly advice you to use OBH, not BH, because of its very effective Fallback tuner possibility.
In the Duo2 the maximum you can have, is 2 tuners. This means that the "maximum" LNB you can take advantage of is a dual output LNB.
As a matter of fact, even if you had a quad output, two tuners can be connected only to two outputs.
This also means that you might have problems using all your 4 receivers at the same time in case you will be watching/recording/streaming from more than 2 transponders.