Season 1 would still be considered the starting season though. If they knew they had 3 seasons to do afterwards 4-2-3 just seemed to make abit more sense. It's still 'standalone' but it just continues off from the previous one better. Of course there would be things that would have been handled differently.Lol nope. Book 1 was originally intended to be a one off miniseries before Nick decided to renew it for another three seasons. Hence the rushed ending as they weren't expecting another season and tried to wrap everything up.
They also intended to have each season be standalone with new villains and plots every season before I guess changing their minds and adding a bit more continuity between the seasons.
How I see it is that after season one, you leave Republic City in turmoil, you still have the aspect of someone uniting them, end with Republic City being at peace after defeating bad guy, ditch the new spirit portal thing, and season 2 and 3 would stay relatively the same. I suppose the first 2 seasons would be very similar and repetitive though.
Whatevs, it's over now. The only good qualities of the show, IMO, was the animation, the beautiful backgrounds and the music.