Help combining ISERROR and CONCATENATE.

I got this last night, and decided to see if I can combine two equations if possible.

Here is what I have:

      A                 B                        C                               D
=RProData!$A15   =RProData!$B15   =IF(ISERROR(VALUE($B7)),$A7,$B7)   =CONCATENATE(4,REPT(0,11-LEN(A1)),A1)

Values returned:

  A       B      C            D
000025 (blank) 000025   400000000025

Can I combine these equations (C,D) into one cell (C), instead of two separate cells?

Thanks in advance guys.