Haven is quads maximum, no allies, and the About page lists it under the things that will not change. It comes up often enough in Discord that it is worth explaining why.
The problem it solves
On a server with no group limit, the biggest group decides how the wipe goes. A clan of ten or fifteen queues in together, takes the best spots, and by the second day every small group has either been raided or is playing around them. Solos and duos stop logging in, the population thins out, and the server ends up with one group at the top and nobody left to fight.
That is a bad week for everybody, including the big group. Rust is at its best when there is somebody worth fighting.
Why four
Four is enough to split the work: someone farms, someone builds, someone roams, someone watches the base. It is small enough that a good duo can still take a fight against a quad and win it. Go much bigger and the gap between a full group and a solo gets too wide for skill to close.
Weekly wipes help too. Nobody gets so far ahead that the server is decided by day four, because on Thursday everybody starts again.
How it is enforced
The limit is about how you play, not about the team button. If you fight, farm or raid with another group, you count as one group whether or not you are teamed in game. That means no shared bases, no "neutral" groups that happen to help each other in every fight, and no calling targets for somebody else.
If you think a group is over the limit, open a ticket with the time and their names. A clip or a screenshot helps more than anything. The full wording is on the rules page.
What it means for you
Whether you play solo or in a quad, everyone you meet is playing under the same limit. If you are short of teammates, the team board is the place to find them: your real kills, deaths and hours on Haven show under your post, so people know who they are teaming with.