blank cells are equal to a zero length string:
=IF(A2="","",A2-A3+A4)
(note: the SUM function was redundant, here)
Another way, if you want to check all three cells, recognizing that COUNT
only counts numbers:
=IF(COUNT(A2:A4)<3,"",A2-A3+A4)

--
- K Dales


"jimtmcdaniels" wrote:

I have a formula adding & subtracting the number values in 3 different cells
into a fourth cell, however I need to change the formula so that if one of
the cells is blank, the fourth cell sum will show blank too.

I think a simple IF formula will work, but I don't know how to tell the
formula if the cell is blank.

example: A1 =Sum(A2-A3+A4)

I've tried something like: IF(A2=blank,"",sum(A2-A3+A4)
but I get an error.