FC2 came with 4.0 on the distro CD we used. We just upgraded to 4.1. Contrary to popular belief, webserver and daemon packages aren't bound to your OS. Your OS does not rely on mySQL, you are therefore free to update to any version that your server can compile. I believe FC2 comes with GCC 3.3.4, the choice is then yours. Go with 4.1, many bugfixes and optimizations over 3.23 - and subquery support!!
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