Different sources show different thickness for the wall. I've seen everywhere from 1.0mm to 1.8mm. I need to know with a high degree of confidence since I'm making an aluminium lego piece of a school assignment, so my wall thickness will be smaller. Because I only get to have one attempt, I can't try it with one piece and adjust the model for a second run.
According to this answer, a 1x1 brick has walls 1.6mm thick - this fits in with the base unit of 0.8mm that is used throughout all LEGO elements, as per pcantin's answer on the same question.
The 1x4 brick in the image you've included shows walls of 1.2mm, while a similar drawing on bartneck's site for the 1x4 Technic Brick with Holes shows a wall thickness of 1.5mm, which is closer to the expected 1.6mm. Curiously the Stud diameter is exactly as expected at 4.8mm.