Incrementing a value in a field

[Mike]

I have a program that contains an order number field. The
current expression that defines the order number (in the
form query) is: "(Chr(65) & Right(DatePart("yyyy",Now
()),2) & Format([OrderIDNewOrders],"0000"))".

Example: A041387

I would like to change the source of the last 4 digits
from "OrderIDNewOrders" which is an autonumber field, to
a four-digit source which I have more control over, so I
can specify the first set of 4. The number should
increment up through 9999 and then start again at 0000.
The order number must be saved with the record so I can
recall it by order number in the future. I've had no
luck. Please help!