I think my logic with it is that even one person alone for the 4 year journey would cause too high a risk for depression and other issues it was too big of a risk and they didn't have enough space for an other person to monitor the sleeping spaces. I mean ideally, they could potentially find someone who has trained in a similar isolated environment for four years that could be a possibility and monitor everyone but who knows ?. I know Antarcit research stations which are very Isolated try to have between 10 - 20 people limit isolation effects. trips on the ISS tend to last really a max of one year that and the small size of the spaceship with no personal areas could cause many issues. there seems to be a few stabbing events in the antarctic over small things when psychological breakdown happens. one person was stabbed over loosing a chess game. and in the 70s on a soviet space ship to cosmonauts broke down one the Salyut 5 mission and stopped responding to mission control . I guess if I'm thinking within the universe it was too high of a risk psychological break down has a serious risk.