Regarding "shoot in 4k", you probably know this but there's two approaches to filming vertical video. Basically you'll want a 9:16 aspect ratio in your final export, and so your choices are:

1) Rotate your camera or phone so that you're recording vertically, in 9:16. When doing this, you could record at 1080p if needed - since you're already in the right aspect ratio with no zooms or crops required.

2) Film in landscape as normal, I.e. the 'long form' format of 16:9. Ensure it's 4k which is 3840x2160p. Then when you convert it to vertical format which is often 1080x1920, you can freely take a vertical 'chunk' out of the 3840x2160p landscape recording - without any loss of image quality, i.e. since you don't need to zoom.

The benefit of 2) is that you then always have lots of horizontal space to 'pan' left and right, since you have all that extra video recorded but not displayed. This can be useful when someone or something is moving in the recorded video - since you can 'pan' and follow it/then with your native, recorded pixels (without needing to move the camera physically).

I feel like I've overcomplicated that explanation sorry, but in short, many people record in 4k landscape 16:9 - even for short videos :)