Hi Traore
I can't answer all your questions, but here are some thoughts that might help.
Firstly, have you checked the next service indicator on the screen behind the steering wheel? This tells you how many miles till your next service and is supposed to take account of driving style - which might cause some variation from the 'normal' service interval of 21000 miles for diesels. I don't know what parameters it uses to assess driving conditions. But from the description you give of your driving, I wouldn't expect it to vary by more than a couple of thousand miles or so. I assume this display is linked to the 'service required' message you are getting, so it might help to know exactly what it is saying, and whether it is consistent over time. Of course, you need to add the miles you've already done to the indicated miles to next service to compare what your car is saying with the nominal 21000 figure. If this reading on your car turns out to be acceptable, then maybe you have an intermittent fault with the 'service required' message you sometimes get.
To see this indicator, press 'menu' on the left hand steering wheel spoke and scroll down to 'vehicle info'. Select this then scroll down to 'next service' and select. Before I had the recall done on my car, this was telling me that I would need a service at 13000 miles. After the fix it now says 19000, which seems OK. When I first contacted my dealership to tell them that a service was indicated at 13000 miles they immediately said that can't be right so it must need the recall I've described. No b... s.... about my driving style, or DPF or anything. So you really shouldn't need an oil service at 4000 miles.
Secondly, if the carbon in the oil really is reading high, even though you don't seem to have had the DPF warning light come on then there is something else wrong with the car and the dealer should be fixing that under warranty, not trying to charge you for a filter change.
Finally, I would hope that when your dealership talks about an oil filter change they are taking it for granted that this includes an oil change - you would hardly drain the oil in order to change the filter and then put the old oil back in. That would go some way to explaining the £200 charge. But I don't think you should be paying this anyway.