Got this from reddit that might help for the moment, it should emulate the *hiressnap function:
How to take 4k screenshots on a 1080p monitor
So simple, you will be killing yourself that you didn't know sooner. Just putting this out there to hopefully stop all these "Free 4k screenshots" posts asking for donations.
Can be done in 5 easy steps
STEP 1 Open Nvidia Control Panel
STEP 2 Go to "Manage 3D Settings"
STEP 3 Turn DSR Factors to "4.00x (native resolution) & Apply
STEP 4 Go to "Change Resolution"
STEP 5 Select 3840x2160 & Apply
-- 24 Jul 2021 11:51 --
For AMD GPU there is something similar but it requires an extra software installed : Custom Resolution Utility (CRU)
1) Start CRU
2) Add a detailed resolution (note that the first detailed resolution will be your monitors standard resolution)
3) Set timing to Automatic - Best for LCD (if you have an LCD of course)
4) Enter your custom resolution and refresh rate
5) Press OK
6) Press OK again to apply your changes and exit CRU
7) Reboot