The recycle program will estimate how much your iPhone 4 is worth if you answer some basic yes/no questions. They offered 75 or something for a 3GS (16 gb) but I think I'll pass up on that offer.
You can probably get more money on e-bay or craigslist but you have to deal with added hassle of customers and payments etc.
I put mine on CL on Sunday, had it sold Sunday night 16g black, almost mint. Tiny nick in the corner of aluminum. $300. I was thrilled. Will upgrade to 32g 4S and be very happy.