Pavel pointed me to this long thread where K1RA seems to have been involved with the FT8 decoder porting and the initial design:
I'm copying K1RA to see if he is thinking a Red Pitaya FT4 decoder might be possible.
Pavel also mentioned that there are currently 38 active users of his FT8 code, combining the tags FT8-Skimmer, Red Pitaya FT8 TRX, and STEMlab SDR FT8 TRX. The software usage stats are at the bottom of this page:
Many other interesting statistics on this page. FT8 is still king but FT4 is growing. WZ7I is the top monitor.