Can I reduce my working hours?

They refused your request. If you only show up for 12 hours of work if you are scheduled for more, it is considered job abandonment or "no call/no show" both of with are fireable offences with no recourse to unemployment. YOur employer has no legal or other obligation to get someone to cover the 4 hours you don't feel like working, and it would be in their best interests to hire someone who can put in the number of hours they need.