I had previously read that you had some health issues in the past; I hope you're doing better since then.
Is there any possibility of upgrading the system to use a newer build of PHP/MySQL? It won't just be the UBB you'll be running into issues with, any relatively recent (within the last 8-10 or so years) PHP script will possibly just "not work" as features have been added to the core of PHP that newer scripts rely on. PHP4 has been EOL for quite some time (PHP5 has been around since 2003 whereas the last release of PHP4 was in 2008) as well.
Out of curiosity, what kind of specs are on the server? My CentOS 6.7 machine is only a quad core 2ghz processor with 4gb ram; I'm running PHP 5.6.17 and MySQL 5.1.73.
