If the company I work for is not supplying me with the hours agreed upon in my contract, do I have to comply with their 4 weeks notice if resigning?

Question Details: About 3 months ago I started a full-time, minimum wage, job. The job was advertised as 'Full-time' and i signed the contract being told directly that i would receive at least 30 hours a week, but most likely 35/40 hours a week. Turns out now Im not getting more than 20 hours a week. Now 4 weeks notice is required if resigning (which I am), since the company is not supplying me with the hours agreed upon, do i have to comply with the 4 weeks notice?

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