Is the absence of childbirth after four years of marriage a punishment from Allah for a sin I committed in adolescence and repented from, and what should I do for Him to forgive me?

Sins are a cause for being deprived of sustenance, yet Allah has opened the door to repentance. Infertility cannot be linked to disobedience, as that is part of the knowledge of the unseen. The questioner should exercise patience and seek forgiveness, as Allah Almighty says: "And said, 'Ask forgiveness of your Lord. Indeed, He is ever Forgiving. He will send [rain from] the sky upon you in [continual] showers. And give you increase in wealth and children and make for you gardens and make for you rivers.'"