Hi, unless there are security patches for either of your software components I would not upgrade. If you are not doing any upgrades to your site and everything works ok then you are fine. I would go for the "if it ain't broke don't fix it approach"
On the other hand, if you decide to upgrade to PHP 5 you would want to do some testing. I would think you would be ok due to backward compatibility but they could have deprecated/changed some things that would require code adjustment.
Good luck with your site!
