What kind of error occurs? What do users experience? Can you access the database using tools and what if you get it back under the 4GB / 10GB limit?
If I remember correctly, when it happened to us, the database just stopped. We had to do a backup to clear enough space to get in and resolve things.
You might like to know that SQL Server 2008 Express R2 now has a 10GB limit, so it might be worth upgrading.