Sayang, ini betul-betul kena jumpa doctor cepat-cepat okay, jangan tunggu. At 6 weeks, kalau tak nampak gestational sac lagi memang perlu check lebih detail, dan a 4cm cyst kena monitor dengan proper scan. A few of my mum friends pernah ada similar situation, dan doktor mereka handle dengan ultrasound yang lebih mendalam, ada je explanation yang reassuring, so don't panic dulu. But you really need to see your OB/GYN to rule out anything serious, confirm your pregnancy is progressing normally, dan check what that cyst is about. Please jangan delay this, book your appointment hari ni if possible!