IF Yes, IF No And ISBLANK Formula

I have a client/sales sheet where I keep outstanding and paid bills information.

I want to get an indicator cell that shows if a bill was paid Y, or Not paid.
Since cell. However, if there is no invoice I want it to show,... nothing.

I used the invoice date cell C4 to see if there is an actual invoice. I thought the following formula would do it. However, if the invoice is paid and the outstanding amount W4 is 0.00 it returns a N, as Not paid????

C4 (date of invoice)
W4 (is amount due - payment =U4-V4)

(this is the formula I thought of)
=IF(ISBLANK(C4),"",IF(W4>0,"N", IF(W4=0,"Y")))