Where did you get that? We've had a number of links to guidelines talking about 10% plus 2 mph (which is what I always thought it was) or 10% plus 1 mph (which is what I heard more recently), and also reports of police forces who simply say, these are guidelines, we don't have to pay any attention, you're nicked sunshine. Not heard about anyone running on 10% plus 4 mph and the wording sounds like it's recent and perhaps temporary.
Still just guidelines. They can do you for anything over the limit, if they feel like it. To certain definitions of "they". I suspect it's a high-up decision, and that individual cops who keep running in drivers for a couple of mph over the limit in an authority which has decided to allow some leeway will be told to stop wasting everyone's time.
You're not "allowed" to do anything over the limit. They may let you away with a small transgression, and they usually do, but it doesn't mean it's not a breach of the limit.