An ultrasound showed a my endometrial lining is only 4mm and there are benign-appearing follicular cysts. could it be pcos?

Not enough info: The dx of PCOS is made by history, physical exam and lab tests. We don't usually rely on us for dx. As always, talk to your doctor. Good luck

Not likely: The diagnosis of PCOS can not be made on the basis of your ultrasound; it requires information on your personal Gyn and General health history plus some specific sonographic criteria and/ or blood hormonal tests. Your gynecologist or reproductive endocrinologist should be consulted to answer your concern.