Use 3840 for both -w and -W?

gamescope -w 3840 -h 2160 -W 3840 -H 2160 -b -- %command% -skipdialog

Instruction:

[gamescope] [Info]  console: gamescope version 3.16.19 (gcc 15.2.1)
usage: gamescope [options...] -- [command...]                                                                                                                                     
Options:
  --help                         show help message
  -W, --output-width             output width
  -H, --output-height            output height
  -w, --nested-width             game width
  -h, --nested-height            game height