MySQL upgrade advices
 

I have mysql 4.0 on my server. The database on the server is critical for me and I myst upgrade mysql version to mysql 5. The server is running Slackware 8.1. What advice can you give me?

Read the MySQL manual. There are some hints in there. I have not done the exercise myself, so can not further advise (except the obvious one: make backups first).

PS:
Just thought of one thing:
A thing I'm aware off is that somewhere in the 4.x series the password function has changed. Version 4.0.23a still has the old function. So you have to pay attention there (the hash is longer and might not fit in the allocated space) and copying the old passwords to the new db will not work (as the hash is different). There is a way to force the old method, but not sure how.