First, this is an extremely naive view as to what goes into training an astronaut to be sent into space. It's not like we just plucked four random people from the globe and are sending them to space. These are people who are very likely at the top of their fields, have gone through rigorous (and costly) training, and are going up to do very specific jobs. These people are not a "dime a billion", and claiming so is the same as saying that if Elon Musk were to disappear today, that any other person could simply take his place and keep moving his companies forward.
Second, just because there are negative aspects in the world that cause loss of life, doesn't mean we shouldn't be doing everything we can to ensure a safe flight. This is unrelated to the cost aspect mentioned in my first point, but the average person living in the U.S.A is only living as comfortable as they are because modern civilization, for the most part, values safety quite high. It's a poor argument to say "Well X people died yesterday, so we shouldn't care so much about the 4 that might die in a launch".