For the record, here's my SP4 experience.
Ever since installing SP4, I now have 2 problems:
1. Every time I shut down the system (click shutdown->Yes) explorer.exe apparently tries to read from a null pointer. The error happens every single time, and always is described as a failed attempt to read from 0x00000000.
The dialog box closes itself after a moment, but its still annoying. I've installed SP4 on 2 systems. I'm not sure, but I don't think this problem has occurred on the other computer. As a side note, it seems like 90% of the application crashes I've ever had using win2k have come from explorer.exe, regardless of service pack version. The explorer team needs help.
2. This one is much worse: my google bar doesn't work anymore. I have tried uninstalling the google bar, and installing the newest version, but that didn't fix it. If I run "Internet Explorer" explicitly, then the google bar is fine, but if I open "My Computer", then there's no google bar and it cannot be made to appear, even after navigating to a web site. This is frustrating since I don't like to use the "Internet Explorer" shortcut - I find it superfluous. The only consolation is the fact that the current version of the google bar does set itself up to be the default search engine, so typing a search term in the address box will work. I have experienced this same problem on both of the 2 systems on which I've installed SP4.
I'm not aware of anything that SP4 has fixed.