Build Your Own SLA/SLS

I need a large build area. So I am looking a better projectors. Some of the 4K projectors list resolution and then native resolution.

One projector for example shows 1920x1080 for native and 2160p for resolultion. Am I gaining with the 2160p, or is it really 1920x1080?

Your not gaining anything unless it's a native resolution of 4K. If it lists 1920X1080 for native resolution that is what your getting.
There are only a few true 4K projectors, I haven't seen one under $9,000. You'd be far better off converging 2 or more 1080P projectors if you need higher res. Problem is the larger you go (I assume you want a large build volume) the harder it is to control the peel/print.