(Source: Southgate ARC)

The new amateur radio digital mode FT4 is now available for download as part of WSJT-X Release Candidate 5 

You can download wsjtx-2.1.0-rc5-win64.exe (or for other O/S) from near the bottom of the page at 
http://physics.princeton.edu/pulsar/k1jt/wsjtx.html

FT4 Protocol document 
http://physics.princeton.edu/pulsar/k1jt/FT4_Protocol.pdf

See the WSJT Group at 
https://groups.io/g/WSJTX/