The iPV D4 looks like it does improve on the D3 to give the mod a better hand feel, and the finishes are very interesting.
The purple version has some antique bronze and black undertones, and the bronze version looks like an antique brass with some black undertones. Neither has a finely machined or evenly brushed look, as the finish is deliberately rough to give it a bit of an old, well-used, even weathered look.
Really, if you already own an iPV D3 and are happy with its performance, and not interested in a marginal improvement in the look and feel, you probably won’t see any benefit in getting the newer D4. That is unless you get a severe case of shiny-itis and must have one.
On the other hand, if you are looking for solid little mod featuring a chipset from a trusted leader, and want something a little different in a new mod, the D4 might be exactly what you are looking for—they really do look cool.