A bit more efficient:

If the only cell that may contain the #N/A is B, then there's no need to
resolve the references and do the addition when checking:

=IF(ISNA(B),0,B)+A+C-D

If the #N/A could occur in other columns, then the proposed solution
fails.

Note that the OP said that D should be subtracted, not added.

In article ,
swatsp0p wrote:

Great solutions, Benjie. May I suggest the OP use the ISNA function
instead of ISERROR? ISNA will only ignore the error caused by the
#N/A, where ISERROR will ignore ALL errors, regardless of the cause and
may hide problems elsewhere in the data.

=if(isna(A+B+C+D),A+C+D,A+B+C+D)