What is the ruling on a relationship that has lasted four years and involved seclusion (khalwa), and how can it be stopped until circumstances are favorable for marriage?

You must repent to Allah and sever your relationship with this girl and all foreign women. This is achieved by strengthening your faith, sensing the greatness of Allah, and frequently engaging in the remembrance of Allah and prayer, as Allah Almighty says: "Indeed, prayer prohibits immorality and wrongdoing, and the remembrance of Allah is greater." You should also frequently read books of encouragement and deterrence, biographies of the Salaf, stories of repentant individuals, and accounts of the horrors of the Day of Judgment and the graves, and seek righteous companionship. As for marrying her, there is no harm in it; rather, it is preferable.