search for a similar recent thread. it was about high water, but you'll get lots of varying opinions.
first off i'd say 4's too young. think about it. would you want him swimming even that kind of water?? even if you're a competent oarsman, always rig for flip. how do you prep a 4 year old for a flip?? take him to a lake.
but if you're set on doing it, PH is the place. and 3 grand is a great level. go right of the needle. go left at the top of yarmony, to right center thru the rest; and get right (behind the backwash of what remains of the hole) fairly quick. the ledges and wall on the left can be pushy. other than that it's cake. maybe 6-7 hours if you row hard, depending on wind, esp in the last 2 miles, the wind after yarmony can be pretty brutal!! might wanna do an overnighter so you can really enjoy it.
be careful with the youngster. but if he makes it out ok he's gonna be hooked!! my first trip was dinosaur on the yampa when i was 6 and it's led to a lifelong addiction!!
All I need are some tasty waves, a cool buzz, and I'm fine. - Spicoli