Update Rollup 5 was just released on July 2nd and is available for download. As with all Rollup Updates, I'm always curious to read through the list of fixes and see if any affect customizations made to my CRM system. It's always a good idea to test out the Rollup Updates for a few weeks on a development CRM server prior to deploying to production. You'll also want to impersonate a test user account to make sure user accounts with different roles and permissions are not affected.
Of particular interest in this Rollup is a fix for "slow performance and high CPU utilization [which occurs] when you import customizations in Microsoft Dynamics CRM 4.0" 956330. Any time I can cut down page load times is a boon to user experience and satisfaction. After installing the Rollup on my dev CRM system, I found page load times for my Account entity reduced from upwards of 30 seconds from a cleared cache, to less than 5 seconds. Highly recommended!