Currency Format With 4 Decimal Places

I have a problem with the currency format with 4 decimal places.

Sub go()
Range("C5") = Range("C5") + Range("C4")
End Sub
as you can see it adds the conecnt of "R4" to what is in "R5"

If works fine when the calls are formatted as number with 4 decimal places.
example if R4 is 0.1111 and R5 is 0.2222 after running the macro R5 becomes 0.3333

But if the cells are formatted as currency and 4 decimal places the then excel returns $0.3300