As per https://learn.microsoft.com/en-us/dynamics365/fin-ops-core/fin-ops/get-started/one-version documentation, "If you encounter an issue on a version that has reached its end of service (N-4), the support team is not able to take your case. You must update to the latest update and then report the issue if it persists.". In that case N-4 version should be supported till Nth release GA + one month for testing and deployment.

Current end of service of each release mentioned in https://learn.microsoft.com/en-us/dynamics365/fin-ops-core/fin-ops/get-started/public-preview-releases does not give support till next forceful release dates.