What's different in 3.2 that requires MySQL 5?

I will leave to others to answer your question specificly, because I don’t know and can’t find anything that definitely will not work in MySQL 4.1. And I may easily have overlooked something.

But you should not expect 3.2 to introduce enhancements or new features that require MySQL 5.1. The decision not to support MySQL was “political”. WP 3.2 is not yet finished, and a feature that require MySQL 5.1 may be introduced any time, in 3.2, 3.2.1 or later.

There has been no priority of the current development cycle to make changes just because one can. Faster, lighter, smoother has been the priority.

But: Support questions and bug reports for a site still running MySQL 4.1 may get very little attention og rejected as invalid.

MySQL 4 is not supported, to have the freedom of not having to check and test things against that platform.

The removal of support for MySQL 4 was announced months before WP 3.1 was released.

MySQL 4 was last updated in 2009. Both it and MySQL 5.0 have reached EOL.

I don’t believe WordPress currently does anything that specifically requires MySQL 5.1, however making it the minimum requirement allows for future versions of WordPress to use faster and more optimized queries that were not possible under older versions of MySQL. WordPress 3.2 is just the “let’s make the switch and strip a few things” while future versions of WordPress will be “let’s rewrite this to be a lot faster now that we don’t need to provide legacy support”.

It’s pretty much the same reason why PHP4 support was dropped — old versions suck. ??

The primary goal of bumping the requirement was to reduce our areas of test coverage. WordPress will currently work under 4.x, but we’re not supporting it, and we won’t guarantee it’ll work in the future.