4k video in 1080 time line in apple motion

Motion is not a video editing software, it is a motion graphics software.

If it is video editing that you are after, neither Motion nor After Effects are the natural choices.

Use whatever works best for you, maybe Premiere Pro is your cup of tea. Remember, though, that Adobe requires a significant monthly payment.

But seriously, the Motion Project has frame size parameters, it knows what to resize to.

In Settings > Project there's a preset to scale "large" photos to the canvas size, but not videos. I'd love to know why it's photos only...

AFAICT, there is no "Spatial Conform" as there is in FCP. The thing is the canvas in Motion is essentially infinite, so I am not sure if this would even mean something.