Can a girl's repentance be accepted if she practiced lesbianism for four years and then repented and regretted it? And is she obligated to inform her fiancé of this, or should she conceal it? And what is the ruling of Islamic law on this matter?

The infatuation that occurs between one girl and another is a reprehensible act. The girl did well to hasten to repentance, and she must avoid the pretexts that could lead to a return to sin. She should associate with righteous women and attend religious seminars. The mercy of Allah is vast, and one should not despair of it. Allah Almighty says: [Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."] (Az-Zumar: 53).

She must conceal her past and not inform anyone of what she used to do before repenting, based on the Prophet's (peace and blessings be upon him) saying: "All of my Ummah are pardoned except those who publicize their sins," and "Avoid these abominable acts that Allah has forbidden. Whoever commits one of them should conceal it with Allah’s cover and repent to Allah Almighty." She must repel the whispers regarding her affair being exposed, and if it happens, she should use ambiguous statements (ma'arid) to deflect the accusation.