Is it necessary to make up the estimated days not fasted from the previous Ramadan, knowing that the estimation was 6 days, but the duration of the menstrual period was only 4 days?

Previous fatwas have stated that whoever is unsure whether she owes the make-up of one or two days is only obligated to make up what she is certain she missed. Accordingly, you are only obligated to make up the four days you are certain you missed, and you are not obligated to make up the two doubtful days, nor are you obligated to pay expiation (fidyah). However, making them up as a precaution and to clear oneself of responsibility is good.