Is it possible to have MySQL 4 and 5 coexisting peacefully on Linux? Some quick Googling isn't turning up anything useful. Since I've only ever installed MySQL on linux via package tools, this will be an interesting experiment for me. I assume that I'd set them up to listen on different ports, so my web server could connect to "localhost" for MySQL5 or "localhost:12345" for MySQL4. Can both be running as daemons at the same time? Anyone ever actually done this?

It highly depends on the packagers. For example, binaries need to be renamed to include version numbers (you'd get mysql-4.1 and mysql-5.0 with mysql symlinked to one of the two). I know for sure that Debian and deratives have packaged it in such a way that you can install both. I don't know about other distro's. In my experience, Suse doesn't other package things in a way multiple versions can co-exist, but they might have for MySQL.