First, use the formula:
=IF(B3=0,"-",B4/B3-100%)
(Notice the added check for dividing by zero.)

Second, select the cell holding the formula and use:
Format Cells Number Custom
and in the Type field put
+0%;-0%

One thing I left out. This is an ongoing chart that I'm inputting
data in periodically and the premade graph updates as I go. With the
new formula, my result now displays -100% for the cells that I
haven't entered in any data for yet.
... can the result still display nothing with no data in cell B4?

One way:
=IF(OR(B3=0,B4=""),"",B4/B3-100%)
but you probably figured out something like that already.