The Dish 500 is engineered for 2 LNBs. If you tried to put 4 LNBs on it, at least two of them would be in the wrong position, and your signal strength would suffer. It is possible to "hit" more than 2 orbital locations with bigger dishes; the Dish 1000 series are examples of that.
What is it you are trying to do? If you want to feed more than 2 receivers, there are DP quad LNBs and DP (and DPP) switches.