How can I act regarding my parents' rejection of my proposal to a girl I have loved for 4 years, and is there a way to convince them after the girl is on the verge of marrying someone else?

A relationship between a non-mahram man and woman is not permissible except within the framework of marriage. Regarding marrying this woman, obey your father and defer marriage to her or anyone else for now, as it is presumed that his refusal is for your benefit. If you fear falling into indecency, then you must marry and not obey your father in delaying it; for obedience to Allah takes precedence over obedience to a father.