He never said that.

He said that Season 2 will end with the Volume 4 and that it will content 3 volumes which was the following:
- Volume 2 = Generations
- Volume 3 = Exodus
- Volume 4 = Villains
After the writers strike occured and Volume 3 (Exodus) got "axed", they changed the last episode of volume 2 and made volume 4 (Villains) to volume 3.
That's all! There was NEVER a comment as about how many seasons Heroes will ever be and in my opinion I think that it depends on its quotes/ratings and how high they are.
The higher the ratings, the most will be NBC willing to buy, the more NBC buys, the more seasons of Heroes will appear!
If NBC cancels the show or doesn't buy any more seasons/volumes, then Heroes will die. It's as easy as that. Shows like Heroes at least can always extend their storylines through new characters, happenings and more. Lost as example was "centered" on the isle and through this they are more bound to the isle so they said from the beginning that their "huge" storyline will go for 8 seasons only but at the end got cut down to 5 seasons (we only saw 3 seasons in germany so far and from what I saw in greece last month the 4th season is already on the run in DVD or something?).