Earlier quoted context omitted.
Did you actually tried it and did it use 4GB of RAM for you? I did and it used about 120 MB of RAM, which is not tiny, but not even in the ballpark of your figure. I know hating Electron apps is the new fad, but it would be nice to check the facts before you grab your fork.
How long did you leave it running? In my experience electronic apps tend to leak memory for some reason. Atom starts up at a couple hundred megs, and actually does end up at around 4GB. Same with Slack, although it's gotten better. I used to actually have a launchd script that killed Slack and restarted it every 30 minutes to keep the memory usage down.
My guess is that compared to Slack, Github app doesn't really do much when idle. Slack fetches user statuses, new messages, notifications and so on. When you have an open channel with ongoing conversations that will add up in the RAM. This app doesn't seem to do anything when idle, so the RAM usage doesn't increase.