Is it worth recording 4K resolution from a 1080p camera input?
I do not know how the choice between 1080p and 4K affects the recording quality in Ecamm live.
But it makes sense to convert a high-bitrate 1080p video to high-bitrate 4K video before uploading to YouTube. In my experience, the resolution increase will significantly improve YouTube video quality. This is because YouTube will use a significantly higher re-encoding bitrate at 4K compared to 1080p. Only a part of the re-encoding bitrate increase is consumed by an increase in the baseline bitrate demand when going from 1080p to 4K. Much of it will remain available for improving the video quality.
Here is as an indirect example a busy frame of a high-bitrate 1080p video converted to high-bitrate 4K video and uploaded to YouTube, and played back using 1080p and 4K settings:

The blocky artifacts in the 1080p stream are much more prominent.