Update buildkite/plugin-tester Docker tag to v4 - autoclosed

[renovate[bot]]

This PR contains the following updates:

Package	Update	Change
buildkite/plugin-tester	major	v3.0.1 -> v4.0.0

---

Configuration

📅 Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• [ ] If you want to rebase/retry this PR, check this box

---

This PR has been generated by Mend Renovate. View repository job log here.