This is your first 'proper' test involving the white Conversion Gel, so one key point to remember is that whatever direction you're facing when you're laying a portal on a surface (such as the floor), is the angle at which the gel comes out. Knowing this helps you direct the flow of gel much more effectively (which in turn makes things a lot less frustrating)!
Keep the portal under the white gel where it is and now set the other portal in the gel outside the enclosed box. Now you're in more open space, turn to face the slanted wall panel at the far end of the room (to your left).
Once the gel has reached the top, place a portal on the wall above the metal walkway and drop down into the portal. Once on the walkway, run around it until you can see the slanted wall below you. Set a portal on the panel then walk back to the side of the pillars.
Aim your portal gun straight at the floor and then fall towards the ground - firing the portal off - and you'll soar through the air and onto the wide open platform. Proceed up the stairs and through the exit towards the fifth chamber.