We only integrate versions of PHP available from Redhat/Centos and their SCL team have only got as far as 7.3. As EL7.9/Centos7.9 is considered feature complete I would have thought it were very unlikely that any more PHP versions will be released.

There is a third party repo, REMI, which has PHP 7.4 compatible with EL7.9/Centos7.9 in two flavours. One flavour is as a direct replacement to 5.4. The other is in a format the same as the SCL versions (PHP 7.0 - 7.3) and with a very minor bit of tweaking (changing the listening port, I think) could probably be integrated into the PHP engines app, but it may require more such as changing the daemon and other bits. We do not normally include apps from third party repos so are unlikely to go down the REMI route.

For Joomla 4.x the minimum requirement is PHP 7.2.5, so it should work fine in ClearOS.