Comp settings? Different res assets in 4k Comp?

Here are a few of the things that you need to consider when preparing a video.

1. 4K footage has 4 times as many pixels as a HD footage.
2. Most streaming services throttle playback frame rate and frame size according to bandwidth. 4K 60-fps renders are only delivered at 4K to viewers with the bandwidth to receive that kind of data. Your uploaded file will be recompressed to at least 3 different frame sizes and the original will never be broadcast.
3. Perceptually, there is no difference between a 60 fps movie and a 30 fps movie because the average human eye has a frame rate somewhere closer to 18 fps. The only real big difference is Motion Blur because the standard effective shutter speed in 60 fps footage is about 1/100 second and a 30 or 24 fps comp is 1/50th.
4. After Effects calculates every pixel on every level. Rendering a 4K source to 50% in an HD comp takes the same processing power as a 4K source in a 4K comp.
5. Previews are rendered and stored at the comp's Magnification Ratio up to 100% if the Comp resolution is set to Auto, and that's the most efficient way to preview any comp.
6. Camera tracking and motion tracking look at detail differences in the pixels. In some cases, pre-composed, scaled and even color corrected footage with Collapse Transformations turned on will track better than the original footage. 

Personally, I seldom render 60 fps movies because there is little or no perceptible gain in quality. Sports programs are sometimes broadcast in 60 fps to reduce motion blur and allow better slow motion replays. Games are rendered at higher frame rates to heighten the edges instead of mimicking the reality of the scene. 

There is very little to gain by working in 4K and rendering HD if your final output is always going to be HD. I work and render production masters as 12-bit ProRez files for all my movie work, and I render H.264 movies for client review or general delivery in HD. When I render tutorials I usually work in 4K so users can get a very close look at the details if they need to.

I hope this helps. Just remember and make sure that twice the bandwidth is worth the 60 fps and 8 times the bandwidth and storage requirements are worth 4K 60 fps.