In a 'normal' 4&1/2yr old boy's chest xray, should you be able to see 3-4mm white-grey round spots thru out his lungs?

Possibly: You may be describing pulmonary vessels on end. The film must be read by a radiologist.

Probably vessels: Ask your doctor to confirm that the radiologist did not see anything worrisome. Without seeing the images, it is difficult to know for sure what you are seeing. That said, a normal chest xray usually has scattered round densities that represent the larger pulmonary vessels "on end." the vessels aim many directions and some are positioned just right to look like round spots.