u/Large-Exercise1040

I’ve recently been offered a new job full time in something paying a lot more and more suited to my interests and something I can pursue a career in , I’m only a 4-8am shopper at Sainsburys 4 times a week. My contract says that my notice period should be 4 weeks even though I’ve only been here since September. Someone else that I know recently left but his notice period was only 2 weeks. Should I just hand in 2 weeks notice? What would happen if I just stopped turning up after those 2 weeks if they expect me to do 4 weeks.