If you were pulled by the Police or had an accident requiring an insurance claim with ‘goods’ onboard with a Class 4 MOT I’m guessing you are not compliant with the law.If you tell the MOT tester that the vehicle is a motor caravan & not used for carrying goods, it will be tested as class 4.
I’ve already heard of cases of the Police prosecuting tradesmen driving on ’social and domestic’ insurance.