There's no resolution related setting in EmulationStation - are you sure you haven't set-up the resolution in your desktop ?
Please post the EmulationStation log taken after starting EmulationStation with

emulationstation --debug

@MegaTimX said in Rpi 4, No display on my 3440x1440 monitor, works perfectly on my 4k TV.:

Hopefully in a future update HDMI support for this monitor(resolution?aspect ratio?) for the Rpi4 will be added.

Unless someone reports it upstream, I don't think it will be magincally solved.
Someone reported something similar this last year and for them it has been solved with a firmware update, see https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=248037&start=25.

If the recommendations in that topic won't solve your issue, then I suggest replying to that topic and adding your own monitor EDID for the RPT engineers to check it.

@microb41 According to the Raspberrypi website, there is a pixel clock limit, ie effectively a limit to the number of pixels that can be controlled, of 1920x1200@60 Hz, so as your resolution exceeds that, it shouldn't work

You can set higher resolutions, in config.txt, but they won't work.

https://www.raspberrypi.org/documentation/configuration/config-txt/video.md

Further down on the same page, it shows you how to create a custom screen resolution, so it would be worth you looking at that to get the aspect ratio you want.