can your boss sent you home after 1/2 hr of work when you suppose to have 4 hrs ?

yes.

if the union contract requires that you be paid for at least 4 hours once you have started a shift, themn you get 4 hours of pay.

otherwise, you are SOL. if this happens often, get a different job.

you may have lost money if you only got 0.5 hours for the day. GAs for the car may well have been more than your net pay for the time.

Most likely yes. Though given absolutely no background info, it's quite impossible to know for sure.

Yes they can. In CA they might have to pay you for a minimum of 3 hours.