The answer is indeed (4).
The rule that after a “to” should come a verb in its infinitive or base form does not apply in this case as “looking forward to” is a phrasal verb.
- He admitted to stealing from his neighbour.
- I look forward to hearing from you soon.
- I look forward to tasting your secret recipe.
- Our staff is dedicated to providing good quality service to our customers.
- I am committed to attending the ceremony.