I need help understanding the hadith that says "...Then if he goes back to it a fourth time, Allah will not accept any prayer from him for forty days, but if he repents, Allah will not accept his repentance and He will cause him to drink from the river of al-Khabaal.” It is known that Allah forgives all sins, so why does it say in this hadith that Allah will not forgive if someone drinks alcohol for the 4th time? There are many people that are addicted to drinking alcohol and are trying their best to abstain from it, but according to this hadith, they won't be forgiven if they drank more than 4 times? Please help me understand this hadith.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu`alayhi wa sallam, is His slave and Messenger.

This Hadith was narrated by At-Tirmidhi in al-Jaami’ on the authority of ‘Abdullah ibn Omar who said: “That the Messenger of Allah said: “Whoever drinks Khamr, Salat is not accepted from him for forty days. If he repents, then Allah will accept his repentance. It he returns to it, then Allah will not accept his Salat for forty days. If he repents, then Allah will accept his repentance. If he returns to it, then Allah will not accept his Salat for forty days. If he repents, then Allah will accept his repentance. If he returns to it a fourth time, Allah will not accept his Salat for forty days, and if he were to repent, Allah would not accept his repentance, and he will be given to drink from the river of Al-Khabal.” They said: “O Abu ‘Abdur-Rahman! What is the river of Al-Khabal?” He said: “A river of the pus from the inhabitants of the Fire.”

This Hadith was classed as Sahih by al-Albani  in Sahih al-Jami’. What is meant by his repentance not being accepted from him the fourth time is an exaggeration of warning and severe rebuke, as mentioned by Al-Mubarakpuri in the explanation of At-Tirmidhi . This is because if a Muslim servant repents, Allah will accept his repentance, no matter how many times he sins, provided his repentance is sincere every time.

Allah knows best.