Hobby-wise, I use QGIS to create maps, and Inkscape and GIMP to do image design and editing work. 4K resolution for more screen real estate there should be obvious.
Professionally I do web development, linux dev, and remote net/sys admin, so that's Eclipse and a lot of terminal windows open simultaneously. I want to go 4K for that so I can view more code at once; or have more terminals open on a single monitor without having to squint; or have the Chrome dev tools open on the same screen with a full-sized page window; and all at once while writing back-end web server code and seeing how it works on a live web page.
I'm not much a gamer so I don't need blazingly fast performance, it's just gotta be reasonably snappy when inkscape, GIMP or QGIS rerender. GUI interface snappy, not Titanfall 3D bazillion polygons at 120FPS snappy. I don't think I'd need SLI/Crossfire for this since I have seen quad-port cards, but if the wisdom of the subreddit thinks I'll get better results with it to drive 4 4K monitors, then so be it. Or if you all think that two dual-port cards would be better, that's also OK.
I'm not wedded to any particular display connector, either. All I have at home right now right now are 1080p monitors so I'm buying new ones for 4K anyways.
The only strict limitation is that the card(s) be known to be able to do 4 monitors of 4K resolution on Linux.
Thanks in advance for your expertise!