What minimum oz. Fluids is ok for my 4yr old?

Varies a lot. A normal 4 year-old who drinks 6 oz at breakfast/lunch/dinner, and 4 oz at 3 snack times, gets 30 oz/day. Drinking more is ok, as long as he is not drinking sugared drinks. Drinking less is ok if his urine is light yellow (first morning pee is darker) during the day. If his urine is medium or dark yellow, he should drink more water. If the child is healthy, a parent can avoid battles over water.
5x/day. I agree with dr kwok, but would add that in my experience, a healthy child whose is well hydrated will urinate 5 or more times per day.Even here there may be normal variation, but this seems to be a good standard.Thus they need to drink whatever amount it takes for this to happen.