What is the ruling on my breaking the fast in Ramadan, thinking that my menstruation had not stopped, knowing that my usual period is four days, but the menstruation stopped after two days?

Upon seeing purity, a woman must perform ghusl and pray, even if it is before the end of her usual period. If the bleeding returns, she reverts to being in a state of menstruation.

Your not performing ghusl, prayer, and fasting on that day when you saw purity was a mistake. You must make up for that day and seek forgiveness and repentance from Allah for your negligence in seeking knowledge. We hope that you will not be sinful for intentionally breaking the fast due to your ignorance.