Yesterday, Microsoft released Update Rollup 1 for Windows 2000 Service Pack 4. Think of this rollup as something less than a service pack but bigger than an update. It contains all the security updates for Win2K that were released between the release of SP4 and April 30, 2005, along with some important non-security updates. Why April 30, 2005? Because that's when the rollup was locked for final internal Microsoft and external beta testing.
If you want to deploy the rollup, you most likely won't need to do much testing before deployment because the number of updates the rollup contains isn't high, and if you've kept your systems fully patched and up-to-date, you've probably already downloaded most of the updates the rollup contains. So should you deploy it anyway? According to Microsoft, the answer is yes: The rollup files contain fixes that weren't included in individual security updates.
Keep in mind that there won't be a Service Pack 5 for Win2K. Support for Win2K will remain at the SP4 level for the remainder of the OS's life-cycle. (Win2K's end-of-life date is no sooner than January 1, 2010.) You're not required to install Update Rollup 1 for Win2K, but Microsoft urges that you do so to keep your Win2K installation current with the most important security updates. On new Win2K installations, install SP4 before installing Update Rollup 1. Continue to run Windows Update for important updates released after April 30, 2005.
To download Update Rollup 1 and access more complete information about deployment concerns and other topics, visit Microsoft's Help and Support pages for the rollup.