K.L.
I found Aveeno baby sunscreen is good. it reads "mild as water". but, it does contain chemical so for a 4mo old I would check with your ped to make sure they agree. get a sun bonnet and take an umbrella or something to keep out of the sun. for getting in the pool, who knows how he will react! my daughter LOVES her bath and so I assumed that the first time in the pool would be great. I think she was about 6 months, she looked at me like i was NUTS. but, the second time in the pool at 7 months she had fun. I guess just go slow in the water, sit on the edge or on the step and splash the water and see how he reacts. let him set the pace.