I have been searching ways of doing it since 3 days now, I couldn't find anything suitable..

what do you recommend to do?

I have a lot of pages running php stuff, I need to update php to 4.3.10 but I dont wanna break what it's working now (even if it's unsecure)

Upgrade the Plesk to 7.5 and after this from the "Update" section update the php to a latest version.