DEFINITELY upgrade! The difference in price between 3 and 4 days is small, and if you don't, it'll tear your heart out on that last day with the park right there and you can't go in.
Sharing meals is a great idea, or taking leftovers back to your hotel if you've got a fridge/microwave in your room. Most portions (except for burgers, sandwiches and stuff) are huge. You could also eat off-site, but the restaurants around the park aren't much cheaper. Again, share meals, especially at places like Rainforest Cafe. They serve up way more food than any one person should eat in a single sitting. Your body will feel better if you eat more smaller meals anyway, rather than one or two gigantic meals that fill you up and put you in a "food coma" for a couple hours.
I usually fill my suitcase with energy bars, trail mix, etc., and eat those for lunch or snacks. It also gives me extra room in my suitcase on the way home. There's a few convenience stores around where you can get sandwich fixins, and you can pack lunches in with you (make sure to bring some baggies or something). PB&J doesn't need to be refrigerated, depending on what brand you buy and how full of preservatives it is, and with the mix of protein and carbs, it's a good balanced snack. You can also buy fruit and veggie snacks at the convenience stores and in the park.
Also, be sure to check with your hotel--most of the ones around the park offer free continental breakfast. It won't be real good, but at least it's something and will get you out the door.