Hi, the config file is located at CastlevaniaAGA_Win64\FS-UAE\Configurations\Default.fs-uae ... open it with Notepad or similar, so you can edit. By default, the FS-UAE embedded emulator runs at 960x540.
So, I don't own a 4K monitor to test but I'd assume that you need to add these lines in that file:
fullscreen_width = 3840
fullscreen_height = 2160
4K displays are 3840 x 2160 resolution. Source and more video options to try: https://fs-uae.net/video-options