we already have 4K option but in .webm format ...
can you add 4K but in mp4 ?

Do you have any problems with using already existing options for format conversion?

Explanation: If you can't find 4K mp4 on the list of download formats, download 4K webm file. When the download is finished, right click the download on the list ans select "Convert to mp4" from the context menu. Unfortunately, it works only for a single file, you can't select more downloads and convert them in a batch.