Crop a video to 3:4, the standard-definition frame stood upright. It is the proportion of a tablet held in portrait and of a lot of print and catalogue layout.
Close to 4:5, and not a substitute for it
At a 1920 px height, 3:4 is 1440 px wide and 4:5 is 1536 px. The 6% difference is invisible side by side and decisive at upload time: a social feed expects 4:5 and will crop a 3:4 clip back to it on its own terms.
Use 3:4 when the destination is a page, a grid, or a tablet layout. Use 4:5 when the destination is a feed.
Product grids
3:4 is common in catalogue and storefront photography, where a column of stills sets the shape and any video has to join it. Matching the ratio matters more here than keeping the extra width, because a single frame out of proportion breaks the grid.
The crop from 16:9
A 1920x1080 frame cropped to 3:4 keeps 810x1080, which is 42% of the width. Fit with bars is the alternative: the full picture lands 1440x810 across the middle of a 1440x1920 frame with 555 px of background above and below.