I want to combine video clips of 4K and 1920 resolutions. Is Shotcut capable of exporting it to .mp4? What export settings are recommended?
Yes. There are several presets to pick from YouTube, H.264, HEVC for the MP4 container (extension). But you can also customize your settings in the Video/Codec tabs in the Export window as well.
It would be hard to recommend settings because there are just endless choices based upon your needs of the project. I publish to YouTube, so I just pick the YouTube preset and don’t worry about.