What is the punishment for accusing an honorable woman of adultery without bringing four witnesses?

Eighty lashes

(Unless you repent and are really sorry.)

those who accuse honourable women but bring not four witnesses, scourge them (with) eighty stripes and never (afterward) accept their testimony - They indeed are evil-doers - Quran 24:4

Save those who afterward repent and make amends. (For such) lo! Allah is Forgiving, Merciful. Quran 24:5