Workshop: Change Waste-Ink-Tank Canon Pixma iP4000: Step 2: Removing the printer´s top
Now you remove the printer´s top. Open the clips in the two marked places, pull the top to the front and lift it off the printer.
The top might resist your efforts, because it can get stuck in several places. Do not use brute force, just bend the afflicted part just a little bit.
Now you can take off the cover of the interface as well.