I think PS4 could pass 3DS lifetime, but only after 3DS is discontinued.

I doubt the 3ds is going anywhere anytime soon. So i'd say yes.

I don't see it happening. It will probably take 2-3 years for the PS4 to get to the current 3DS level, and by then 3DS might be anywhere between 65 and 90 million.

Nintendo will discontinue the PS4 long before it comes within spitting range of the 3DS's lifetime sales. My uncle who works at Nintendo told me so.

Nah cant see it catching it before its discontinued

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.

How can Nintendo discontinue PS4?

OP means the 3DS

I doubt it, but it is possible! I will remember this post and watch.

Why should Ninty discontinue 3DS? Outselling its competitor 5:1 LTD and 3:1 weekly in the first half of its life after their respective launch (and relaunch ) peaks is a thing that won't happen again easily, 3DS domination on PSV is what should matter for Ninty, not being reached by PS4. Also, if portable market is really shrinking due to siege by phones and tablets, being a longer lasting platform, and so allowing longer term projects to SW devs too, can be a selling point for portable consoles, that won't ever be able to compete with phones and tablets on the performance race.