Chapter 4: The Surprise
Test Chamber 18
After your surprise, enter the chamber and portal across the gap to the platform behind the turret. You'll find a button which releases two Redirection Cubes which collide and fall into the goo. To prevent them from falling, shoot a portal on the ceiling above the light bridge, and one in front of the far cube deposit tube... or whatever you want to call them.
The cubes will land on the other side of the gap, where you started. Portal back to that platform, and get one of the cubes to the laser - you can use the light bridge as a shield if you need to. Redirect the beam through the two visible switches. Grab the other cube, and use it to direct the laser at the turret, destroying it. Direct the laser through the rest of the switches, and use the bridge to reach the exit door.
Test Chamber 19
The first Redirection Cube you'll need is easy to spot on the floor of the chamber. Grab it, and portal up to the platform across from the laser beam. Redirect the laser into the first switch (on the other side of the chamber).
The second cube is on a high platform near the entrance - you can reach it by placing a portal above it. Once you have it, place a portal on the floor, and one above the Faith Plate. Drop the cube through the floor, and the plate will launch it back out. While it's in the air, fire a portal onto the wall across from the first switch (which the laser now runs through), launching the cube onto the platform. Do the same for yourself, and redirect the laser to the second switch.
The third and final cube is on another high platform near the entrance. Use the Faith Plate trick to launch yourself out of the wall perpendicular to it, knocking it to the floor. Portal the cube and yourself up to the platform which the laser beam is now aimed at, and use the cube to direct the beam into the turrets guarding the exit. Once the turrets are destroyed, aim the laser at the final switch. A platform will be raised, allowing you to use the Faith Plate to shoot yourself out of the wall and to the exit door.
Test Chamber 20
Shoot one portal at the spot where the laser beam meets the wall, and another in front of the three switches. Use the Redirection Cubes to aim the other two laser beams through the portal and into the switches. That's all there is to it.
Test Chamber 21
Fire a portal onto the small strip of wall in the far left corner of the room, and another at the end of the bridge. Walk across the bridge and through your portals to access a button. The button will drop a cube onto your bridge - walk toward it and - oh no.
Wheatley will open an escape route for you. If you want an easy Achievement/Trophy, fall for GLaDOS' trick (you will die, but you won't be set back far). Otherwise, run down the light bridge and continue the escape. You'll encounter a room of turrets. They're pretty easy to dispose of, and once you do, you can portal out of the room through a gap in the impromptu walls. After that, there's only one more turret to take care of, and a portal below it and on the wall next to it will do the trick.