Sisters praying in masjid, we can’t see the men. We sat for tashahhud after the third rak‘ah. We did not realize until the imam had gone into rukū‘, and some of us had not even finished reciting al-Fātiḥah or stood up in 4th rak’ah. What to do in this situation?

Falling behind the imām is of two types: 1. Falling behind due to an excuse. 2. Falling behind without an excuse. The first type is when it is due to an excuse. In this case, he performs what he fell behind in and follows the imām, and there is no blame on him, even if it is one full rukn (pillar) or two. So if a person forgets and loses focus, or does not hear his imām, until the imām gets ahead of him by one rukn or two, he performs what he fell behind in and follows his imām. The exception is when the imām reaches the position this person is in. In that case, he does not perform it. He stays with the imām, and one rakʿah (unit of prayer) is valid for him, pieced together from two of his imām's rakʿahs: the rakʿah in which he fell behind, and the rakʿah that the imām reached while this person was still in his place. End quote. Al-Sharḥ al-Mumtiʿ (4/186)