Quickly shoot the second portal under your feet before you touch down. At the same time, you have to be directing your fall so you land on the 3-dot platform. As you fall, shoot a portal across to the 4-dot floor, aiming for the left side of it. There are some blocks sticking out of the side wall that can queer your shot here, so watch out for those. Get into a position where you are facing towards the 4-dot platform with minimal obstructions. Shoot the second portal onto the walkway where you can move around it and look through.To the right of it, you should be able to see a bit of the adjacent higher ceiling. Go out to the end of the walkway and over against the wall on your left.