How to select best N list from a pool of M lists having to get the maximum number of unique elements?

Question

For example, I have 7 lists as shown below. These lists can be of length l>0. `

L1 = [1,2,3,4]  
L2 = [1,2,3,4,5]  
L3 = [6,7]  
L4 = [1,6