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

Yes.

Your boss can send you home whenever he or she likes, but you are entitled to "reporting pay" which in Ontario is 3 hours.

In Ontario, most employees are covered by provisions of the Employment Standards Act which create, in essence, a minimum shift. There are myriad exceptions to its application, but the gist of it is that an employee is usually entitled to a minimum of 3 hours pay at minimum wage.

Unfortunately yes