Version 5.4 no longer supported

[stevef]

Is it still safe security wise to use version 5.4 ?

All software has vulnerabilities.  The patches issued to supported software help to minimise exposure.
Using unsupported software increases risk of exploitation as the hidden vulnerabilities are exposed but not patched.

How do versions 6.4 and 7.4 differ from version 5.4 ?

Best to start by reading the release notes.
https://www.linuxliteos.com/forums/forum...php?fid=11
If you have any specific questions, please ask.

It looks like the website no longer has a shop that sells the USB boot drives for later versions ?

I believe the shop section was present until the website redesign in August.

So a download from the website is the only option to get a later version ?

A direct download (or torrent) should work, there may be other ways - avoid buying media from auction sites.