The PS4 sales in Japan are probably irrelevant if they already started the developement. If they decided to develope the game for PS4, they would have to change the platform right now. I'm not sure that this is something that is just done "like that" in an instant.
Maybe this is already mentioned, but i think they already confirmed that the game wont be on mobile at least.
Why would they have even started a PS4 title? DQ games don't tend to come early in a system's life, they need time to build an install base.
3DS is not only the system with the biggest install base, or the one with the best odds of software selling several million, it's also the most mature current gen system.
Anyway the fact that DQIX was originally an action RPG and got completely revamped tells me that they would in fact be willing to go back to the drawing board.