I think the OFFSET function will do it for you. Yep, that's what you want. Try this:
Code:
=SUM(OFFSET(B3,0,1):OFFSET(B3,0,4))
The OFFSET function starts with a given cell (B3, in this case, which is the very cell the function is in), then adds a certain number of rows and columns to it. I specified 0 rows and between 1 and 4 columns.
You can just as easily specify a different starting cell and then correspondingly different row and column offsets; this just seemed intuitively obvious to me.