How to stimulate brain of 4 month baby & what to teach him?

Hello,

I have a son who is 4 months now. I am already showing him flashcards for A to Z & shapes.

I wanted to ask that, How can I stimulate his brain further? What more can I add here?

Is it advisable to show him youtube videos for infants? Can you please share if any?
Or hardcopy of books will be better choice?

Please guide me if any specific online books are available that I can take print of.

Also please guide me, when can I start “little reader” to him.

Regards,
Mayur