Q: when I play a 4 players game, have I, when I build the star players deck, to take ALL the star players in the game (so the star players of the 2 teams not in play too) or simply the star player cards of the 4 team in play?

Ex: teams in play: Orks, Empire, Dwarves, Skaven. Star player decks (one OWA and one CW as stated in the rules): O+S and E+D star player cards only (+freebooters of course) or WildElves and Chaos star players too?

Yes, you put all the star players + freebooters in their respective decks.

In your example, the Elf and Chaos SP would also be included in the decks.

So, if I'm right, I can draft any Star Player, even SP of a different race (OWA and CWC decks limits remain, of course). Am I right?

Yes. Correct you can draft any star player in your respective Conference deck regardless of race of the player. And you have access to the star players of races from that conference that are not actually in play - hence the little tokens to ID the players.

Thank you guys... Tomorrow night I will play my first game!