Is there an easy way to save a contact for a phone number in the text of an SMS (iphone 4s IOS7)
Since you're running iOS 7, all you can do is copy it and manually add it to a new contact.

Update to iOS 9.2 and you will get the option to do exactly what you described in your post. Don't worry, even your iPhone 4s should run iOS 9 fine and updating may even make it faster.