How many seasons NCIS:LA has? And is Kensi,Deeks, Sam and Callen in all of them?

Season 4 starts in the US on September 25, Season 1-3 are available on DVD (at least in the US). As for the characters: Callen, Sam and Kensi are in from the beginning, Deeks had guest roles in Season 1 from episode "Hand to Hand" on, since Season 2 he's in the opening credits