Reading a few posts on here and seems loads of people dont actually understand the insurance classes.
May be a good idea to get a post running that can clear them up.
This is my take below. Please update if incorrect or you have additional info.
Cat A is for burn outs and gutted vehicles. Only value is in the weight at a metal recyclers.
Cat B is where the vehicle is no longer safe to put back on the road and must be broken or crushed. These can only be sold to registered dismantlers with an EPA waste license. They are actually sold as parts and are no longer considered as a vehicle. There is no economic consideration involved, purely a safety one. Vehicles that have been flooded are normally written off as a cat B.
Cat C is where the car is repairable but is not economic to do so. Think older cars and or ones where the chassis needs jigging.
Cat D is where the vehicle is repairable and would be economic to do so (cost of salvage + cost of repairs < % Pre accident value) but the insurance company has decided not to repair the vehicle. This could be due to excessive storage or recovery costs, hire car cost totals etc. Most cars will be cat d'ed if the cost of the fix/hire car/storage/recovery approaches 50% of the cars retail value.
So in terms of a 4k car with cat d its most likely something like a bumper and 2 headlights will cause it to get that classification. My mates £1.5k vectra b got a cat d'ed for a door. He bought it back for £600 notes and paid £40 for a door. Cat d's can be good value as long as you know what has happened to it. See plently of pics before and check it over well when looking at it.