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I am pleased to say that you do indeed pass the "4 year rule".
You should therefore make an application for a Lawful Development Certificate.
Under the Town and Country Planning Act 1990: Section 191 as amended by section 10 of the Planning and Compensation Act 1991.
Town and Country Planning (General Development Procedure) Order 1995 the following applies in your situation-
A Certificate can be obtained where over four years ago, there was a change of use of a building, or part of a building, to use as a single dwelling house. Enforcement action can no longer be taken once the unauthorised use has continued for four years without any enforcement action being taken.
I hope this assists and answers your question.