What should I do with the 400 dinars my father gave me to give to my neighbor as aid for the death of his son, which I did not give to him, and four years have passed since then?

The trust must be returned to its owner. Disposing of it unlawfully necessitates repentance. If you cannot pay the amount, you must inform its owner of what you have done, or inform your father so he may absolve you of it. Hasten to do this before it is too late. If you fear shame, you can use subtle phrasing when responding by having another person deliver it to him.