First you need to check if there is a problem with the router. If the network Settings on the iphone4s are incorrect, you will not be able to connect to the wireless network. In this case, you need to make sure that the wireless LAN Settings for the iphone4s are correct.
Sometimes, the iphone4s can't connect to wifi because of a problem with the operating system. In this case, you need to try updating the operating system. This is how the iphone4S can't connect to wifi.