There's a hidden room with a puzzle in secotr 4. Simply complete it, and you get the trophy.
The entrance to the room is not very well hidden, and once you're in the room it will become clear what you have to do. Those who want to find it for themselves, turn back now. Those who want help can click on the spoilerised information.
*** Spoiler - click to reveal ***The hidden entry is in the second puzzle room of sector 4, the one where you have to jump to a ledge using two blue blocks.
*** Spoiler - click to reveal ***Once you're on the ledge, don't continue forward but look back into the room. On the right, at the same height of you, you'll see a short hallway. Jump to the hallway and walk into it, you'll find the back wall is a fake one. Now you've found the puzzle room.
Open the next spoiler if you want an explanation as to what you need to do in the room.
*** Spoiler - click to reveal ***Once you activated the puzzle, you can see through colored glass at the back of the room. The room behind the glass contains a pair of quadruple red bricks at the end, a pair of quadruple red bricks in the middle and a pair of double red bricks in the front. All these pairs of red bricks are combined with turn buttons.
The point is to line these red bricks up with the colors on the window - the sides of the red bricks are colored as well. Use red bricks on opposite sides to form one whole line, and line up these lines of red bricks with the correct colors on the glass.
Once you've correctly lined everything up, you'll hear a sound to acknowledge the solution. You'll immediately get the trophy and you can continue back into the main room.