Early 2013 Macbook Pro with 4K: Picture quality? This is sort of related to this question. The 13.3 inch Early 2015 Macbook has a resolution of 2560 x 1600. I'm not an expert on displays, I've done a lot of Googling but don't seem to understand.

How does my Retina Display with resolution 2560 x 1600 show up on a 4K screen with resolution: 4096 × 2160 in terms of quality of image? What will the pixel density on my monitor? Let's say the monitor I'm after is specifically this: BenQ EL2870U 28 inch 4K HDR10 Gaming Monitor, UHD 3840x2160, FreeSync, 1ms Response Time, Eye-Care, Brightness Intelligence Plus, HDMI, DP, Built-in Speakers. 


Is my picture quality going to be fantastic? Or am I making a mistake trying to fit 2560 X 1600 into 4096 X 2160, 27 inches and it's going to be super grainy with me being able to distinguish between pixels? 
What are the monitor specifications that I should be looking for to get the BEST picture quality
Assuming that I may upgrade my laptop and simply end up getting a CPU later on in the future, and money isn't my biggest concern, can I just get a 4K monitor and downscale the monitor's resolutions to 2560 X 1600?


The purpose of buying the monitor is not primarily gaming per say, but I would love to get the same FPS that I can on my laptop when playing Dota 2 and less lag during the data transfer from HDMI or whatever appropriate cable. My laptop supports Thunderbolt 2.
From Everymac


  This model supports a simultaneous maximum resolution up to 2560x1600 on two external displays via Thunderbolt. Alternately, it can support a single display up to 2560x1600 via Thunderbolt and a single display up to 1920x1200 via HDMI.


2560x1440 native on a 27" screen is just fine for my old eyes - I've got 2 of them here (screens and eyes ;) so 2560x1600 ought to be OK, assuming it scales down nicely.  

"Best" picture quality would theoretically be a native resolution of that, not scaled - but that wouldn't be future-proof; so you'd have to make that call.