I also started to have diuted fruit juice to my son around 4 months.
I use heinz baby apple juice and add water (1:3) to him.
He loves it and drinks 4-6 oz a day.
my little boy even drink more milk than b4, but some nurse tells me that better to feed him juice /water at least 45 mins b4 the meal time, cos the water/juice can be digest within 45 mins ..