Excel Help - Super low priority but I can't be productive

This is super low priority but I can't be productive until I figure this out. Bassically what I want to do is set up my cash flow inputs to automatically deduct the 3 & 6 month cash flows from 9, or 6 from 3 etc. Aside from doing it manually, going from right to left, or setting up a suplimental row beneath your hard code is there any way to have the cell set up to take the raw number from the user & automatically subtract the appropriate cells? So I guess:

function("user_#")=Q4-Q3-Q2-Q1