The current expectation on the mailing list is 5.4 should be generally backwards compatible with 5.3. Incompatibilities with 5,3 are listed in the manual:
http://www.lua.org/work/doc/manual.html#8
Build testing:
This was created and tested on Ghost BSD (11.1-p4). I have a clean VM for 11.1 and a TrueOS VM (Some version of CURRENT) that I will test this on. I'll pull out an arm board if I can.
Lua Testing:
My scripts are scattered though so my testing of Lua itself will be rather light. I'm also without applications to test. Any suggestions would be great.