Make sure you move the dish angle up slightly to compensate for the weight of the quad lnb
The weight of the lnb has a tendency to pull the arm down a little more than the single lnb does.
If the dishes elevation is set up badly it could make the difference between you getting a signal or not getting one.
Make sure you mark the dish first before you do anything so you have a reference point to go back to if anything goes wrong
Nano