depends on what you have in your database. if it was me, I'd just go ahead and try it... FIRST, do a full database backup and save the file somewhere safe, then go ahead and downgrade.
if everything works fine, you're just peachy, but if not, just upgrade back to mysql 4.x and if you need to, you have a database backup that you can restore. Most applications if they are more than a year old or so, (and not specifically written for mysql 4.x) will run just fine on mysql 3.x as well...
only issues I've seen are if you have anything using the database type innodb, or if you are using any of the newer table types that only work in mysql 4.x
all the applications that I currently run, run on both 4.x and 3.x equally well.