That is correct, you can do it like that as well, but a big issue that arises with this approach is that most computers cant run SFM at that high of a resolution. I have a 2060 super and ryzen 7 3700x, and SFM is very laggy at those resolutions. I have two sets of launch settings, one for normal work, and one for rendering.

For render:
sfm_shadowmapres 8192 -sfm_resolution 2160 -w 3840 -h 2160

For work:
sfm_shadowmapres 8192 -sfm_resolution 1080 -w 1920 -h 1080

This works, but there is a better and faster method

open properties for SFM
put sfm_shadowmapres 8192 -sfm_resolution 2160 -w 3840 -h 2160 into the launch settings
now whenever you render, export as a movie
change the 720 resolution to 2160, the seconds to frames, the selected frames to custom, and change the 1440 frames to 1
rendering with max samples at 4k will take about 30 seconds depending on your computer but it'll be a lot better than rendering as a poster.

I want it to be known that I'm not trying to attack OP with this. I just want to spread valuable information for those that don't want to or can't export posters at this resolution

or just render 1 frame

did that

try adding this into your launch options on steam for sfm:
sfm_shadowmapres 8192 -sfm_resolution 2160 -w 3840 -h 2160

i did that but it still didnt work

you need to make sure the format is PNG and the # of samples is 1024

sfm legit doesnt let me put in a custom resolution value that high

Maybe try exporting a lower resolution photo to upload, and then keep the 4k one for personal use?

You can try saving it as a .jpg/.jpeg