Yesterday, Microsoft released a Community Technology Preview (CTP) of SQL Server 2005 Service Pack 4. You can read about the release on the following blog post from SQL Release Services:

http://blogs.msdn.com/b/sqlreleaseservices/…preview-available.aspx

If you don't want to read the blog post, you can go right to the download pages:

SQL Server 2005 SP4 CTP

SQL Server 2005 Express Edition SP4 CTP

The build number is 9.00.4912.

Note that the CTP only includes SP3 CUs 1-11.  So if you've installed CU #12, installing the CTP may undo some of the fixes from that update.  Also if you want to see the list of fixes that are in the service pack, you can see the following KB article; however, note that the article is clearly not complete at this point and will likely be updated (or replaced with a different article) between now and the official release of SP4:

KB #2303003 : List of the bugs that are fixed in SQL Server 2005 Service Pack 4 CTP