How do 4% of developers get paid to work on Open Source?

In most of these cases, they're not working on it full time. Their company uses an open source project as part of something else, and the developer fixes bugs or adds features as the company needs those changes.

There's a much smaller contingent where the open source project is the core of a closed-source project (e.g. Red Hat employees contributing to the Linux kernel or Fedora) or where a company runs an open source project and charges for support contracts (e.g. MongoDB).