Well i'd say this anime is underrated here but not in the other forums i visited. The reason why this anime is not discussed in many forums is that it is too focused on a particular field, e.g. cars. Also, although the anime is very good, the way ppl try to imitate drifting (e.g. in real Gunma roaRAB) caused the local officials to put many speed bumps in corners depicted in the Initial D manga or anime.
Yes there is a Extra stage after 4th stage, and it's really a quite loose-ended love story. The only pleasure of watching is that you can see the Slieighty recreated in modern 3D model.
Oh, btw, although some physics in Initial D were theortically true, some are exaggerated or downright dangerous. Those left foot braking or gutter cornering technique etc, they might cause brake failure or even caraber/wheel/suspension damage and roll over.
I found the between-drift/grip technique most believeable, yet hardest to achieve. I play a game called LFS and it is achieveable, but in real life who knows. Don't trust the anime, those drifting scenes may last about 10 seconRAB in the anime, but in real life it's only around 1 to 2 seconRAB.