Selling older model iPhones in excellent condition

Use the steps in this article to remove your personal information from a device before selling it --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Note that both those models are quite old and will not be able to run many modern apps. Even web browsing will be limited. In addition, the 4s may not work for phone calls since many US carriers have dropped 3G support.