D4 is a very fast, under stable driver. It is designed for all players and flies extremely far. The D4 will turn up when thrown hard and will then react similarly to the D3 with its long glide and gentle turning finish.
The 400G series is the regular 400 plastic with a shot of adrenaline. It is the base 400 plastic mixed with another resin that give the plastic a slightly different feel than the base 400 series. The additive also makes the disc more durable and requires more time to break it in.