[Hostboard]

First PHP 5.x is EOL as of 1/2019 and no longer support or maintained.

PHP 7 life is shown here:
https://www.php.net/supported-versions.php

The move of your host to PHP 7.3 is very logical.

From a performance perspective you would be amazed at how much faster PHP 7 is that PHP 5 (yes it is that noticeable).

While VB 4 does run under PHP 7.1 which is also EOL, it will limp along on PHP 7.2 but even that is reaching EOL.

Porting VB 4.2.5 to PHP 7.4.9 is really much much less work than you think, and you wont loose your work or abilities offered by the addons. In fact you do not even need a vBulletin developer to make the port. Just someone who knows PHP and can make the necessary adjustments based on the log file. While I am not condoning this there is a copy of VB 4 that is fully coded to work under 7.4.x floating in the wild (not hard to find).

I have been running under PHP 7.4.9 for a few months with zero issues. I am also running MariaDB 10.3 and will be moving soon to 10.5. The next major hurdle will be PHP 8.