I noticed a prominent blue vein running vertically down the center of my 4 year old son's chest tonight that has not been there before. He was very tired from an active day. The bottom of his foot hurt too. Not sure if related or if I should be concerned?

A vein at the surface doesn't have to mean anything serious, nor does a sore foot sound related. But it's impossible to answer this online.
I would put in a call the your doctor's answering service rather than going to the ER on a Sunday morning. Keep your son close to watch him today. See if he can tell you what he might have done at least to hurt his foot.