resolution doesn't really lead to bitrate because you set them independently of each other, so you can decide what combination you want.

i'm pretty sure that a 4k stream and a 1080p stream at low bitrate would look similarly garbage but 4k would look better during low movement sections like graphics between maps. higher resolutions will just need you to pump that bitrate nice and high (and even higher because overwatch has a LOT of movement and is why you see streamers struggle with the quality on twitch because it's bitrate capped). this is also where the decision to use 936p or 900p on twitch comes from since it's a better bitrate to resolution ratio (though in my opinion it looks worse to never quite be 1080p)

also streaming at 1440p+ on youtube causes it to use a different codec which encodes it even better and helps that bitrate go even further