Subtotals depending on columns

Hi, 

I want to create subtotals calculated depending on the user selection - based on 4 columns. 

Any idea how I can achieve that? 

Exemple: 

Level1        Level 2         Level 3          Sum 

A                   abc.              abc.de          1000

I want a subtotal Level 1 - if user has selected Level 1, Subtotal for each row with Level 2(if user selects level 2) and so on. 

Thank you