Site claim their insider hinted Mafia 4 is coming at Summer Game Fest
Czech website Vortex, that did have contacts at Warhorse (they made this documentary) and old 2K Czech, claims they got hints from insiders that Mafia 4 will be 2K game annouced at SGF. As you maybe known Dan Vavra from Warhorse, was lead designer of first Mafia at 2K Czech.
In Czech:
Podle našich informací je ale na spadnutí představení nové Mafie. Už dříve jsme dostali několik náznaků od insiderů ze zahraničí. V posledních týdnech se o odhalení Mafie čile mluví i v českém zákulisí. (...) Vlastně by nebylo překvapením, kdyby se studio Hangar 13 ozvalo ještě před červnovým Summer Game Festem.
Translated:
However, according to our information, a new Mafia is about to be unveiled. We've gotten a few hints from insiders abroad before. In the last few weeks, the Mafia reveal has also been talked about behind the scenes in the Czech Republic. I won't put my hand in the fire for that, but if I had to choose, I would point my finger at Mafia. Actually, it wouldn't be a surprise if Hangar 13 studio made an announcement before the Summer Game Fest in June.
They conclude, that they doesn't have 100% proof. But few people from 2K Czech were moved to Hangar 13 studios in US.
Vortex had some real leaks in the past - they have been hinting at annoucement of strategy game Last Train Home (they also had small roles in the teaser) and were fisr to annouce openning of GSC Studios (Stalker Devs) in Prague (here).