FCPX - Can't export 4K project as mp4 in project resolution

Hey, I have a project in Final Cut Pro X with a resolution of 4096x2304 (25 fps).

When exporting it using the standard master function for a regular mp4 export, I can however not select the project resolution. FCPX only gives me the option to downsize to 3840x2160 or less, which is incredibly inconvenient and frustrating.

So how can I export the project in a 4096x2304 mp4?

Thanks.