Retrieve month from date

I am using PGSQL 8.2.3 on Windows XP.

I have a table called "StudentFeesPayment" with columns "ReceiptNo" and
"ReceiptMonthYear".
The column, "ReceiptMonthYear" stores date in the format "yyyy-mm-dd". I
have to find the max(ReceiptNo) where Month of (ReceiptMonthYear)=4. Or
whatever month I give.

I tried:

Select max(ReceiptNo) from StudentFeesPayment where
Extract(Month('ReceiptMonthYear'))=4;

But it is not working. How to do it?