If anything posting this survey just shows little OP (and the folks over at ResetEra) understands about basic PC terminology. Primary Display Resolution is your Windows Display resolution.

There is a reason why not many people have their Windows display at 4k. I'll show it in a picture.

There. See that lovely Windows 10 (I hate it, 7 for lyfe) GUI on a 4k resolution? Can barely read the text. It's worthless. While yes, there are ways to make the display 'normalized' it's much easier to just set it to 1080p since it's so much easier on the eyes. Hope y'all console folk learned something!

To be fair, I play mostly on PC and Switch and I didn't even know that. I don't understand these steam surveys sometimes... they lack description for the criteria (or is it on a different page?) For example with the 1080p thing I see that it says Windows, Linux, or Mac. So right away we know we're talking about operating systems. But we don't know if that could just be users who game on a Mac machine or a Windows machine. The resolution could still be referring to games. But I know it isn't now, I just seem to forget the criteria every time a new Steam survey pops up.

You can change the size of text and things in settings though. I think. I'm not at home to test it. 

What do you mean it's easier to set it to 1080p? Literally right on top of resolution is display scaling, it takes no longer to change scaling than to change your resolution to 1080p.

Primary resolution isn't the resolution that someone uses in game.

I highly doubt that.

1) That's display resolution, not rendering resolution.

2) 1.33% is still millions of PC gamers.

3) 1440P is the "sweet spot" in terms of resolution/price/hardware performance across all platforms anyway.

Correct, You can... But it's extremely terrible, especially on Pre-Windows 10 Operating Systems.
Not as much of an issue over on the Mac side though.

This. As for me, I'd personally rather use 4K outside games and use 1080p in games, just because I get more space to work with in 4K but I don't need it in games. I do need the better-running games though, which is why I'd still be tempted to use 1080p in games even if I had a 4K monitor (which I don't, because I don't feel like getting a ridiculously expensive graphics card which I'd need for games to look decent on that monitor).

JFYI it means native 4K, so there is like 0 people playing in this resolution on PS4.

Around 30 percent of the Steam users could use "console 4k" settings (30 fps, dynamic resolution 1200p - 2160p)... any PC with a 970/1060/290/480/580 GPU or above.

Most of these people seem to prefer higher fps than 30