All that the 4-year rule confers is a defence in a possible enforcement action.
An effective defence, though, surely? Assuming, as you say, that it can be proved the development is more than 4 years old.
Cheers
Richard
Yes, it would be a complete defence to an enforcement notice, though the burden of proof would be on the OP.
In practice, of course, it is highly unlikely that the council would be potty enough to initiate enforcement procedings in a case like this. If they lost, they would be liable for the costs of the enforcement appeal (asuming that the OP had initially made it clear to them that he could prove that it was > 4 years old).