I'm not denying they have 4000 teams using the product, but YC has an internal tool, so getting YC companies on board which have multiple teams per company, I think you'd reach 4000 quite quickly.

This isn't to take anything away from the app itself, but with almost 1500 companies funded by YC, and access to more connections and marketing...well, you get the point.

How big are these teams? How active are they?

4000 teams of 1 that used it for a couple of weeks before abandoning is way different than 4000 teams of 100 that are active everyday.