There comes a point when a timeless design becomes...timeless. Every revised version afterwards is (frankly) more of a marketing exercise than offering anything different from the original. Hence we end up with silly 'Maserati' and 'Anniversary' versions etc.
Unless they have endlessly deep pockets, few existing D2 or D3 owners are going to jump to the D4 revision.
For those of us with 'lesser' speakers sitting on the fence wondering if we should go B&W, then hopefully existing stocks of D3s will drop to more affordable prices. For example, the current advertised price (without haggling) for a 803 D3 is c. £13,000, compared to £16,000 for the new 803 D4. Now if the 803 D3 drops to well below £10,000...