Recently I stumbled upon a file with .lz4 extension, wondered what that
was and if Emacs could read it.  It turned out out that it has been
compressed with a shiny new very fast compressor, found on
https://cyan4973.github.io/lz4/.  Fortunately its commandline syntax is
quite standard, and so it was straightforward to add jka-compr support
for it, see the attached patch (tested on the emacs-25 branch, but
jka-cmpr-hook.el on the master branch is identical).

For those who want to test it and not build lz4 from source: on Debian
and derivatives, the lz4 program can be found in the liblz4-tool package.

Cheers,
       Sven