Did your manager tell or ask you to leave earlier than supposed to? Or, did they tell you that you are no longer needed? You may wonder about your rights as you were expecting and also expected to work from/to a certain hour.
You can politely decline their request/proposal and tell them you want to fulfill your contract. The contract is binding for both parties.
You are guaranteed four hours of pay and this must be settled as soon as the disruption occurs. The contract terminates with immediate effect when you leave the premises so make sure your compensation is settled before you go, or before you accept to cancel your shift. This is applicable to those who have worked less than four hours or those who are told not to come to work after all.
If you are asked to leave because of a breach of contract, as in - you did not have the required skill for the role, conflicts caused by you, lateness, etc. then the employer has the right to send you home.