It depends on two things:
a) What Nintendo is planning for the 3DS' successor
b) The true trajectory of the PS4
If we look at Nintendo's hardware development history, they tend to ramp up development in their handheld/ home console successor right after the current one peaks. Which means they're probably at the drawing board as we speak.
If they decide to go their usual route of "innovative hardware design" and create something with an auxiliary feature (say VR), then they could be able to release their next handheld as early as Christmas 2016 due to using only in-house EAD development. If decide to develop a more a complex device to combat the growing popularity of smartphones (say a Phone-DS hybrid), they might need to restructure and expand their assets which would mean that we couldn't expect a successor until late 2017 at the earliest.
I'd usually expect them to just develop a traditional handheld, but with the continuing struggles of the Wii U, the release of the 2DS, the CEO's plans to revise the company's focus, I'm not so sure. I guess we'll just have to wait and see.
As for the PS4, it's definite that it will do very well but exactly how well is uncertain. MIcrosoft could give tough competition (thus costing the PS4 significant marketshare) and the PS4 could end up ~110M. If the hype continues, Microsoft exhausts all of its cards and the PS4 does not bomb in Japan, then the PS4 could be on a trajectory to dethrone the PS2.
Though I think people are somewhat underestimating the PS4 and overestimating the 3DS, the only way I can see this necessarily happening is if Nintendo drags out the 3DS and the PS4 continues it's record-breaking trajectory.
