What I see in this habitat is this is not a torture like living on Mars would be under confined conditions. Firstly, they should have larger crop areas and make them only eat what they grow. One shower and one toilet for all four would present an area of edicate needed in such conditions. Everything else is Ok, but the structure itself, should be tested for air tightness. So that the entire thing can be reproduced on Mars without having to do the whole experiment again for any of it's aspects. My only other comment on this is, with the kitchen and the rec center so close together, they'll eat and sit around so much they'll become like fat pigs! A larger fitness area would help this problem.