What is the wall thickness of a lego brick?
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.