Is there any ruling in Islam that a baby's feeding of food to be commenced by a function (commeration) at four months, four days with dates and fish or it is an innovation?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There is no basis in Islam for a given celebration after a period of four months and four days following the birth of a new baby. Whoever celebrates it as a religious rite, in fact, practices an innovation.

According to the Sunnah, one can chew a date and then put it in the baby's mouth right after his birth.
For more benefit, please refer to Fatwa 82088.

Allah knows best.