Upgrading From Mysql 3.23.x to Mysql 4.0.x

[getDizzle]

I am thinking of upgrading to MySQL 4. Currently we have about 12 very large production sites that all use MySQL to a great degree on Redhat 7.2. Would upgrading to 4.x break any sites using MySQL based on the 3.23 build? We use everything from the most basic of select up to very large join queires. I'm just curious if someone has upgraded with a similar setup and had any problems.

Also, I see that subselects and unions are only part of the 4.1 alpha build, is there any better support in 4.0 for complex join queries than in 3.23.x?

Hope all this was clear enough to get an answer 🙂

Thanks!