Version 5 is currently in alpha testing, and we would hope soon be in beta testing. Of course how long, will depend on that testing.
In terms of there being an "issue" with supporting a higher version, its simply that we havent tested nor developed the product for that version, and we have seen things break. At present the focus is on development of the new version 5 release. As PHP 8.1 is supported for security releases, there is no immediate focus to support this on version 4.
I would just like to comment on this part. We are currently in a major release cycle. As such, the latest version of PHP is supported in the next version available for us to feasibly do it in, as it always is. However as this time its a major release, it of course takes longer to develop, test and release.