Got the saved-all-butterflies ending, then went back to get to the secret room in chapter 4, which took several resets because I kept flying into the cameras >.< The secret room just goes outside and lets you explore OoB, which AFAIK has nothing in it. Flying up from the exit (or around the entire map) leads to the victory screen, at which point you can win by flying to the trigger near the normal exit, but nothing happens except you are told you didn't find the butterfly exhibit. You could also bring butterflies out with you this way, at least the one in chapter 4.