Hadith - Al-Tirmidhi #3643, narrated Abdullah ibn Umar ; Abdullah ibn Amr

Allah's Messenger said, "If anyone drinks wine Allah will not accept prayer from him for forty days, but if he repents Allah will forgive him. If he repeats the offence Allah will not accept prayer from him for forty days, but if he repents Allah will forgive him. If he again repeats the offence Allah will not accept prayer from him for forty days, but if he repents Allah will forgive him. If he repeats it a fourth time Allah will not accept prayer from him for forty days, and if he repents Allah will not forgive him, but will give him to drink of the river of the fluid flowing from the inhabitants of Hell."

Some people say its authentic , others say its not so I don't know but I do know that Allah forgives us no matter how many times a person commits the same sin as long as repentance is sincere so how come, if the Hadith is true, it doesn't go for a person who drank a fourth time?

I know that part of repentance is staying away from that sin however for all other sins Allah will still forgive because He knows we are only human and will always sin.. So just because a person repeated it doesn't necessarily mean s/he wasn't sincere.