I like multis and I think discoveribility is a bottleneck, but I'm very wary of this idea. If you merge communities together like this, you essentially multiply the users in that community. Moderation isn't 4 small instances anymore - it's one large one with 4 separate mod teams each handling a quarter of the posts

I think this is more likely to lead to polarization and eventually echo chambers than if you kept them separate - outrage drives engagement more than anything else, and explosive growth is a great way for a fraction of the group to dominate the first few pages of comments, which turns off moderate voices, which works like confirmation bias to make the outraged believe they're the prevailing voice of the community, which again drives them to post more incendiary comments, and the whole thing spirals

If you want to avoid echo chambers, the best way is to throw a small group together and make them get along through mods that are involved in the community

But then you'd probably end up with most members of one community slowly joining the rest, which is a healthier growth model, but still not great

My intuition is that the ideal solution involves encouraging users to join a single smaller group, but being exposed to top posts from sister groups to avoid fomo. Possibly through something like the way Reddit handled crossposts, where you get the post but not the comments, and a small link to the discussion in other communities. It could be automated if the post crossed a certain threshold of votes, keyed to a certain deviation above the daily average of the original group and optionally with a minimum up/down vote ratio.

This would help keep moderation ahead of participation, and hopefully build a tighter knit community - people are less willing to be jerks to people they recognize than strangers you get in a larger population. By encouraging users into one small random group instead of shopping around for the one that best fits their view, I think we could resist natural grouping by beliefs.

To go further, if this works we could consider a mechanism for "mitosis", a splitting of a group when the mod team feels the culture of the group is getting past their ability to manage in a nuanced way

The goal is decentralization after all, not distributed centralized groups