What are possible implementations (or examples) of the four-eyes principle?
One of the implementation on code is the Pull Request model (PR)  popularized by GitHub.

The main reasoning behind is that only a small set of maintainers of the product will be allowed to merge code into the release branch. Every new feature/bugfix will happen on a new branch and once done will be defined as a pull request.

This allows to test on the merge from actual release (master) code with the code in the PR in an automatic way (Travis being the most popular for public project actually) and give a first feed back on the code quality. Travis CI (for example) can run on the result of actual master with the  code from the pull request merged into it, so it will fail if the merge is impossible, or if the commands defined in the travisci.yml return a non zero exit code

Once the automatic tests have passed, for the 4 eyes principle, it still requires a configurable number of persons to review and approve the change before it gets merged obviously at least 1 person (who is not the PR author) to enforce 4 eyes reviewing the changes.

There's a wide range of options to merge automatically once the quorum of reviewers is met or to still need a manual merge by a maintainer.

The permissions to review and merge can be separated which helps giving more people the right to "vote" on the merge status while keeping the possibility to restrict who can actually do the merge.