Is a young man considered unjust towards a young woman if he leaves her after a four-year romantic relationship, based on an unfulfilled promise of marriage, and what should he do to please Allah and the young woman if he has wronged her?

You must repent and sever the relationship with the girl because it is a religiously impermissible relationship outside the framework of marriage. What is required is to cut off the relationship, not merely reduce communication. If it is possible to complete the marriage with her and fulfill your promise, then that is good, for the Prophet, peace and blessings be upon him, said: "Nothing is seen for two who love one another better than marriage." If that is not possible, then there is no blame upon you for turning away from marrying her, as fulfilling a promise is recommended but not obligatory, and breaking off an engagement is permissible for both parties.