Three of the "invisible walls" are near the beginning of the game, but the fourth is way at the end, so if you've missed one of the first three, you'll have to go back on a new playthrough.
The first is at the end of the initial Peru tutorial level. Before you have Ben pick up the cancer-curing flower, use Dan to try to run off the cliff at the right side of the screen, and an invisible wall will stop you.
The second is in the London apocalypse level. After finally getting Dan to give Ben a piggyback, you'll have to put Ben down so he can open a door. Once the door is open, Dan will say something like "I probably shouldn't put you down for this next bit...." Go to the right through the door, and there will be an explosion behind you. Turn around and try to go back to the left through the door, and you'll hit an invisible wall.
The third is at the beginning of the Joy level. After Dan jumps over the first tall platform, Ben will be killed and sent to the respawn area for some good old-fashioned puzzling. Switch back to Dan, and try to jump back over the tall platform (you'll need to get a good running start and time the double jump to get on top, but it's doable). Once on top of the platform, try to jump off to the left, and an invisible wall will stop you.
The final invisible wall is way at the end of the game, when you go back to Peru to experience Regret. Once again, before Ben interacts with the flower, have Dan try to jump off the cliff, same as the first time, and the achievement should unlock.