There is nothing wrong with running 5.3.x, that tree is still being given security updates depending on your flavor of linux, there is no need to upgrade to 5.4.x at this time.

IB is sad for not addressing vb 4 series long ago, heck, hey still havent released a stable version of 3.8.x for 5.4, they still list it as beta lol

If you have shell access to your server, then this thread may help you out:

vBulletin 4.2.x and PHP 5.4 unofficial work around automated shell script

Forum Search is not working as well.

Well, I guess I do not have much of a choice here.

--------------- Added [DATE]1377497529[/DATE] at [TIME]1377497529[/TIME] ---------------

Update : Just been informed 4.2.2 will be available in about 2 months and would be working with php v 5.4+.

This is good news!

Update: turns out one of my tables was crashed searchcora_text. I repaired it and everything is working fine as it was. Now I am using VB 4.2.0 PL3 with php 5.4 without any errors.