Match names between two sheets and return value of a cell in the row

Hi there:  

I am tracking forecast and actuals for various projects and vendors (example attached).  I'm looking for a formula to compare the projectname, services co. name, month and year on different rows and then subtract the amounts in column N (net amount due) from column P (forecast amt.) - if the projectname,services co. name, month and year matches.   I have thought about concatenating the four criteria into one long wordstring in a new column to compare, but now sure how to use this to subtract two other columns if the workstrings match?    Any help would be greatly appreciated!  Thank you,