I'm note sure in what kind of format these exercises in your book are written...? Can you actually see if those particular notes are meant to be played open? If it's regular tab, it could be notated that way, but may be the result of a typo or wrong interpretation by a computer program which generated the tabs. If it's standard notation, the positions are chosen at the player's discretion. Note that you take into account the ease of the fingering, the previous and following notes (context) and the desired "tone" of the note to decide that.
You speak of chords in particular. It could be that you need to finger the note at a different position. But that wouldn't make much sense for the book, letting the student figure that out by himself, rather than simply pointing it out. The only thing I can think of is that the "open" string with the 4 with it should be
muted with the pinky. That's about the only sensible thing I can come up with if the "4" does indeed indicate the fingering. Hope that helps!
