Resolution is how many pixels make up each frame — 1920x1080 for 1080p, 3840x2160 for 4K. Because it is a two-dimensional count, halving each dimension quarters the pixel count: 1080p has 44% of the pixels of 4K, and 720p has 44% of 1080p.
It is usually the first lever to pull, for a reason that has nothing to do with encoding theory: most video is recorded at a higher resolution than it will ever be viewed at. A 4K clip that will be watched in a chat window, embedded in a slide, or scrolled past on a phone is carrying pixels nobody will see, and they are expensive.
The published bitrate ladders make the saving concrete. Working from YouTube's and Apple's recommendations, pulled down for re-encoded rather than master footage, this site budgets roughly:
| Resolution | Comfortable | Acceptable |
|---|---|---|
| 4K | 20,000 kbps | 12,000 kbps |
| 1080p | 4,000 kbps | 2,000 kbps |
| 720p | 2,200 kbps | 1,100 kbps |
| 480p | 1,000 kbps | 500 kbps |
So dropping 4K to 1080p cuts the data requirement by about 80%, which is more than any quality setting will give you without visible damage.
The rule worth keeping: match the resolution to the destination, not to the source. Downscaling is cheap and mostly invisible. Upscaling is never useful.