It does what a number of the big name projector companies have been doing... Pixel shifting the image. The panels are "just" 1080p panels, but through a bit of magical trickery, it can shift pixels around to give the illusion of 4K. A true 4K projector in the same class would run about 50% more (which isn't a small price at about a $5000 and up increase).
Imagine rapidly "shifting" the entire 1080p panel rapidly enough that you can fire one full set of pixels, shift a tiny amount, fire another set, shift again, fire again, and so on... all within a fraction of a second. That is how some (if not all, I just don't know) are doing it.
Here is an article (from 2013, so the projectors they talk about are a bit dated, but the tech is the same) about it: What is pixel-shifting?
You can YouTube videos about 4k pixel shifting. And while some reviewers love the idea, others say that it isn't worth it and you should wait for true 4k.