How much should a 4ft 5in 11 year old weigh?

Depends . . .: We use height-weight charts to plot growth curves. As long as a child follows a curve, then s/he is growing appropriately. However, if s/he crosses a curve or two, then we worry. For any specific age, height & weight, check out http://nccd.cdc.gov/dnpabmi/Calculator.aspx for the CDC's calculator for BMI (body mass index) percentile for use in children & teens.