To make something lawful like an extension (which required planning permission when the work was done) the 4 year rule applies ie after 4 years it becomes lawful and the council can't do jack.
Development never becomes lawful just by passing the 4/10 year limits. It remains unauthorised, and the only way to legitimise it would be to apply for a certificate of lawful development.
The council may not be able to take enforcement action after the time limits expire, but there still may be issues with insurance/mortgages etc if work is classed as unauthorised
The time periods for enforcement are national and not discretionary by local councils (four years after the initial breach in the case general development, and ten years in the case of changes of use or breach of conditions) - however the decision whether to take enforcement action is discretionary