Q: Two devices, one apple id account, want to restore only one and sell

When I upgraded from my iPhone 4 to my 4s, I activated it using the most recent backup file from my previous device. My old phone no longer has a carrier service, but is still recognized in itunes in respect to my apple id. I now want to sell the iPhone 4, and need to restore it to factory settings. How do I perform this task without affecting my current device, my 4s? I am inclined to assume that it will only remove the data/media from the 4, but I want to make sure I am correct before I unintentionally erase all of the info from both devices, which would very definitely cause me to have a stroke. Thanks in advance.