Cut fruit looses nutrition? Not significantly.
Cut fruit looses about 10% to 25% of its vitamin C and other oxidizing nutrients after 6 days in refrigeration; it visibly spoils before nutrition loss is significant. Most oft-cited paper with this result is Quality Changes and Nutrient Retention in Fresh-Cut versus Whole Fruits during Storage (American Chemical Society), you cans find a brief summary from the NYT Nutrition desk here: http://www.nytimes.com/2006/06/06/health/nutrition/06nutr.html, and a google search for "cut fruit loses nutrition" has top results mostly pointing to that study. An argument can be made for pre-cut vegetables and fruits being less tasty after a week of near-freezing refrigeration, but this "4 hours" thing is malarky.
Vitamin C reacts with "metal" to become useless? No.
I found a few articles discussing vitamin C reacting with free iron & copper in a H2O2 (hydrogen peroxide) solution to become a free radical; but these conditions would never be present in the body, and certainly not in your orange juice. Orange juice mixed with hydrogen peroxide would be waaaaay to gross to drink. Ew. I couldn't find anything other than this Yahoo answers question that dealt with the question directly.