Your PR status does not disappear automatically. If you've been out for four years, you are not compliant with the residency obligation, which can lead to the PR status being revoked.
Assuming no valid PR card, to return to Canada as a PR, you'd either have to request a PRTD to enter Canada, or enter through USA. In both cases IRCC could start the process to revoke the PR status. This would be highly likely in the PRTD case and fairly likely if enter through USA; in both cases you'd have a chance to make case for humanitarian and compassionate considerations to keep your PR status and appeal.