Where does the 4k come from if a bike is already type approved?
Oh, I see what you mean - that's the cost of the bike. Well, they're not cheap even in Germany, and possibly you might well be subsidising the cost of type approval.
In any case, you would probably not want to go there for lots of other reasons. There's a fee for registration, and third party RTA insurance could cost you £100 or more per year. The tax disk would be free but would need to be displayed. When the bike is three years old there's the annual MOT probably at motor cycle rate. Then there's the somewhat large reflective yellow number plate, which would mean you can't use cycle paths, you can be 'done' for exceeding the 30mph speed limit if you tear down a hill, you'd have to be careful at traffic lights, nipping up on the pavement to get around roadworks would be a no-no, parking in London would be a problem (certainly in Westminster) - etc.
In other words, most of the disadvantages of riding a bike, most of the disadvantages of riding a powered two-wheeler - and few of the advantages of either.
Just my opinion, of course!
Rog.