Theres no incompetence at present Avoid unnecessary invasive procedure Instead start medicine to avoid complications

It is better to undergo a cervical encirclage as ther eis history of previous loss of pregnancy at 20-24weeks which is typically the time for pregnancy loss due to incompetent os...also you should continue progesterone

Progesterone support and bed rest , is what you require

According to your history, Cervical  insufficiency   is more apt  a diagnosis and I would advice you to have a encerclage done at the earliest. Presently at 20weeks your cervical length being 4cm is good . But an incompetent  cervix  can suddenly shorten and cause problems.  Since you already have had 2 traumatic losses please  go ahead for the procedure along with progesterone supplementation.

As cervical length is 4 cm at 20 week I don't think incompetent os was reason for your previous abortion. Progesterone is good enough

If it is  a  proved Incompetent  cervix  better go for  cervical  cerclage.