Personally I have no problems with screen door efect even if my viewing distance is 3m away from 120 inch screen but there are people out there with much better eyesight.
So again, these projectors rely on good quality optics in order to achive good sharpness. TW9400 it's a very sharp projector and it can resolve 4k details very decent despite 3LCD limmitations in this regard. On the other hand DLP projectors are very sharp and can resolve very good 4k details because of DLP technology thus allowing for a cheaper optics.
If you take, for example, the optics from OPTOMA UHD 50/51 and you put it on Epson's projectors, the image will be utterly trash becouse 3LCD doesn't do well with cheap optics.
On the other hand if you take the optics from TW9400 and you put it on a DLP, you will end up with something like BenQ W11000 , which can resolve 4k as good as a native 4k projector. But still the on/off contrast is only half of Epson's 9400.
From what I have read, the image projected by TW9400 is the pinnacle of 3LCD technology in regards of everything, from black levels, native contrast, sharpness, etc. (I will leve out of disccusion the proffesional, very expensive projectors used in movie theaters, becouse those cost a fortune).
Epson is the only major player that has hold on this LCD technology and invested a lot of money to be able to deliver good products out of it. Others have gone for DLP , which is the easy way. We are fortunate becouse otherwise now we could not get decent black levels, beautifull vivid colors and good native contrast without paying a lot of money for LCos projectors which, let's face it, many of us can't afford.