Bioshock 4, or 3? (BioShock 4)

What part will be next in the bill, because the extreme, and, give boh, not the last, was not numbered and the plot it can be considered a subspecies of spin-off or novels on the universe of the game ... Or these previous parts were spin-offs...
Will it be the new DDR 2 in the style of original retrofuturism, or will it be possible to play on the road on the tablet? Does it make sense to wait and will the next part come out in principle?