My cervix was 4cm at 20 weeks and water bag broke at 21 weeks cervix was 2cm. is it incompetent cervix? can we go full term with incompetent cervix?

Highly suggestive: Your presentation is suggestive of a loss of cervical integrity. The regression back to 2cm occurred because the bag of water broke. PPROM at 21weeks in the face of cervical dilatation, unfortunately, carries a poor prognosis. However, subsequent pregnancies can be have very favorable outcomes when treated with progesterone and/or cerclage and serial cervical ultrasounds.