Click on a picture and you will get a sequence of short and long pulses (sort of like a star twinkle). Remember the sequences and their position (top left, bottom right, etc.) and head back to the knocker door. A short pulse means click on the knocker with your left mouse button, while a long pulse means press and hold the left mouse button for a second, THEN let the mouse button go (you will both see a different message from "<system>" and hear a different sound). When a knocker's sequence is correct, the background color will change from orange to red. Do that for all four knockers.
If you are still stuck, just do this:
Top Left: Long, Short, Short, Long
Top Right: Long, Long, Long, Short
Bottom Left: Long, Short, Long, Short
Bottom Right: ...I'm not telling - I want you to figure it out :)