The original game does not natively support widescreen or 4K resolutions. You can fix this through the in-game console or configuration files:
Download the or the ISO files from community-vetted sources like Archive.org . unreal-tournament-2004-download-for-pc-softfiler
Unreal Tournament 2004 (UT2004) has recently entered a "renaissance" as a community-supported title, now officially sanctioned by Epic Games to be hosted and patched for free by the fan community. The original game does not natively support widescreen