Some high-DPI support is already in the client, so let me just note my stance here in case anyone is listening:
Render the 3D viewport at full resolution or I will stop developing on Roblox.
There is a specific reason I have a 4k display and that is for pixel density. Rendering the viewport at a lower resolution and scaling it up just completely defeats the purpose of having a 4k monitor. And introduces blurry scaling artifacts that make the game look disgusting.
So please, render the 3D viewport at full resolution.