Where can I live in Philadelphia for 4 months?

Your best bet is going to be subletting from a group of Penn students who rent a house, but have a roommate who's abroad for the semester. Depending on location, you might find a room for half of what you're thinking. I would look 40th to 46th, and Baltimore to Walnut. Lower numbers = proximity to campus, but higher rents. http://philadelphia.craigslist.org/sub/3345490202.html, but it was the first thing I noticed that would get you your own place. Shares would obviously be cheaper.

Under $800/month is entirely reasonable. If you're okay with a bunch of roommates, you could pay considerably less. West Philly is definitely where you want to be. Supercres' street boundaries sound about right. Another word of caution about being close to campus though--in addition to higher rent, being further east (lower numbered streets) means being closer to frat houses/campus oriented bars, so it can get noisy on weekends and holidays. St. Patrick's day, in particular, is a clusterfuck in the areas nearest to campus, but my block, about a mile away, was as quiet as ever. If you have a bike, consider bringing it, or even getting a cheap one here; Philly's very bikable, and getting to Penn from within West Philly is a breeze. http://philadelphia.craigslist.org/sub/3292096476.html is super close to campus, ridiculously convenient to public transit and far enough south on 40th street to avoid the noisiest bits. (Friends of mine lived for one summer at 40th and Spruce, about two blocks north of this listing, and had problems with the noise.) Feel free to memail me if you want to know more about West Philly, or want to check if a particular place listed in a craigslist ad is okay.

Philly is eminently livable without a car. The blue line (aka El, aka Market Frankford line) and trolleys all go through Penn's campus. From there you could easily live in West Philly, Center City, South Philly, Northern Liberties, or even Fishtown and have a half-hour commute or less. Be wary of sketchiness of course, but don't feel like you can only look at West Philly. Also, take look at http://ihousephilly.org/student-housing/. It's not super glamorous, but it's just off campus at 37th and Chestnut, very safe and reputable, fits your price range, and does short-term accomodations.

That's a totally reasonable price range and you can probably get into the $600 price range with roommates. If you get the internship, you can get a temporary account for access to the search function at http://offcampushousing.upenn.edu/.

The area around penn is actually one of the more likely areas you'll be able to find housing. I recommend padmapper. Don't go too far west if you're not bulletproof

Yeah, definitely run a check on Craigslist for sublets in West Philly. My boyfriend is moving out of his place in a house next weekend and I know there is another empty room in it as well; there's a pretty high turnover rate lately, though he lived there for several years. He pays well under $600 for a room and use of shared common areas. He's down by 49th Street which is a little farther from campus but totally safe, and he managed a commute to school just fine.