See pages 22-25 of the front-panel I/O connectivity design guide: http://files.domcxem.ru/infocenter/%D0%91%D0%B8%D0%B1%D0%BB%D0%B8%D0%BE%D1%82%D0%B5%D0%BA%D0%B0/Intel_FrontPannelsGide.pdf
You'll need to source a 3.5mm jack with an isolated switch, not the kind you've shown where the switched...
Possibly the TV just reports all resolutions above native as 4K.
Why the weird resolution? There are only 4 common resolutions for TV panels (1280x720, 1366x768, 1920x1080, 3840x2160) and 2048x1536 isn't one of them..