For this calculate the individual potential energies of every pair of charges and add them.

 

 

In this case,

 

PE due to adjacent charges + PE due to charges due to pair on opp. side of diagonals.

 

4kq²/r + 2kq²/r(2)^1/2   

 

kq²[4+(2)^1/2]/r

 

Hope u hav got it !!!!!

 

Do tell me in case of problems.