According to Nagoshi the Yakuza team's size made him decide to focus solely on Ishin. The team "owed it to PlayStation fans" to have Yakuza Ishin as a PS4 launch title.
Yakuza Ishin is a spinoff of the main series, taking place during the late 1800's Japan during the Restoration, replacing modern Tokyo with market towns, riverside farms and nascent cities and pagoda-style castles.
Check out the Tokyo Game Show 2013 trailer in the video below.