Then what happenss if the immigration officer reported that ge has been out of the country for four years and he has only one year left in the PR ?
Look the basic rule is that PR cards expire PR status never expires but can only be revoked or renounced.
If someone has been out of the country for 4 years since initial landing then they have not met the residency obligation so should they return now they might get reported at the border resulting in the process starting to revoke PR status. This may or may not happen there is no way to predict .
If it happens then as a PR an individual is still entitled to enter the country and appeal any process initiated to revoke PR status which might take a year, again no way to predict but they can stay until any appeal has been concluded.
If somehow despite being away for 4 years out of initial 5 since landing someone enters successfully without being reported then they need to stay in the country for minimum 2 years without leaving at all. They should keep any interaction with IRCC to a minimum, if possible none at all, so no renewal of PR card and no attempts to sponsor anyone.
It is not illegal to not renew a PR card , just having an expired card means travelling outside of Canada can be a challenge as either need a PRTD or return via US land border.
In any case any individual once back in the country should not leave the country whilst the residency obligation has not been met. After the initial 5 years the RO assessment becomes a rolling 5 years so 730 days in the 5 years preceeding any new entry into the country.