Artifacts first, then you can mix in Missions, Horizon and Boardroom as far as being published first and the stuff in Artifacts is supposed to happen before the others time-line wise. However, tossing a bunch of brand new characters into Artifacts can easily lead to a bunch of dead brand new characters. I'd advise you run maybe half of the Season 4 missions first, or all of Season 3 to get them experience before diving into Artifacts.