Use Cells For Start & End Formula Reference

I'm using this formula (in Cells D4:D10) to tabulate hours being spent on various testing activity categories (N,O,R,E,U,A,D). Is there a method I can use to permit me to change the range of cells being tabulated using Named Ranges or some sort of substitution?

= SUMIF($C$15:$C$89,C4,OFFSET($C$15:$C$89,0,4))

for example I would like to use the value I present in cell A2 as the starting point (for the range) & B2 as the end point (for the range) of cells being evaluated. In other words the improved formula would look like this.

=SUMIF(A2:B2,C4,OFFSET(A2:B2,0,4)) -or something like that-

See attached for reference!