I have a machine using a 9500GS. The w/u's take 48 hours. I mis-calculated and let a w/u run that was going to be late by 6 hours. I let it run, it uploaded and was ACCEPTED and I got the credit.
Maybe the 4 day deadline has a grace period. I don't know but there is at least a 6 hour window. Just thought I'd let you know in case you get in the same situation.
Buster is my dog. Sadly, Buster is gone.
Buster is the Malamute, Obi is the Golden.
The WU will be re-issued to someone else if you can't return it within the deadline.
However, if you return it (and it is valid), before the re-issued unit is returned you will be granted credit and the re-issued unit will be cancelled by the server.
This means you have a period of grace after the expiry of the WU of a minimum of about 6 hours (in the case of the WU being re-issued to a GTX280 cruncher), but in practice you may have around a day or so (where more likely, the re-issued WU sits in a cache and is then crunched by a 8800GT).
I've had a few cancelled by server WU's, none while crunching though - only ever when they are in the cache. I don't know what happens if the re-issued WU is already being crunched....maybe you both get credit?