Make sure you know how to get to and from the airport on public transport, because that can be a big hidden expense. EWR will cost a little more to get to/from, although if you're really pinching every penny, you can take a local bus between the airport and Newark Penn Station, which will save you about five bucks each way on the commuter train cost.
Speaking of pinching every penny... that's already a really cheap trip. More than half the cost is plane fare, and you're not going to get it much cheaper. For the rest, yeah, avoid paying for crap food, but treat yourself to a bagel or some Puerto Rican food or something.
Also, there's no budget line for activities. No liquor, museums, attractions, clubs, concerts, tours? Do you have four days' worth of free stuff lined up? Or are you just gonna hang with your friends and talk?