PhpFox 4.80 supports PHP 7.4
https://docs.phpfox.com/display/FOX4MAN/phpFox+4.8.0#space-menu-link-content
Go to your AdminCP, then click on the Maintenance menu, and then click on the menu called System Overview. It should be listed on the top of the page what version you are currently using.
My version does not have a system overview. I downloaded the version in early 2015. At that time phpfox had their copyright on the script. I wanted it whitelabel so I removed it from the code so potential competitors would not have a heads up. I think that it is a later version of 2 or an earlier version of 3 as it does not have the PF.Site/PF.Base file sturcture. I was hoping maybe somewhere in the code they may have it or maybe they would have a version history we could refer to from phpfox itself?
My concern is how to I upgrade from the old file structure to the new file structure, without massivly breaking things? The site itself is clean without any addons. I rely on the website as a member management system that I rely on for a living. Killing the site for even a short period of time could have a significant financial impact on my income.
If it was me...I would go to the source which in this case would be phpfox. It may cost you, but hopefully, it would be a lot less than what you would lose if attempting to do it by yourself and then have to hire them to fix any issues later.