iphone 4S - my wifi keeps dropping or it comes up with a message "unable to connect"

Use this article: http://support.apple.com/kb/TS1398

Is this on all networks, or only your home network? If it is just at home reboot your router by removing all power for about 15 seconds.

Restore from backup. http://support.apple.com/kb/ht1766 (If you don't have a backup, make one now, then skip to the next step.)

Restore as new device. http://support.apple.com/kb/HT4137
For this step, do not re-download ANYTHING, and do not sign into your Apple ID.

1. Did this just start randomly, or have you ever been able to connect to this network?

2. Does this phone connect to other wifi networks?

3. What is the strenght of the wifi connection when you are close to the router (1-3 bars)?

Try to isolate and see if this is just an issue with just this network. If so, and you have access to the settings on the router, I would test out with no security or a different chanel. http://support.apple.com/kb/HT4199 gives all of the recomended settings for wifi networks so that the phone should work optimally with that network

If you are having this same issue with all wifi networks and have gone through a full restore of the phone to new, then you likely have a hardware issue with your phone and may need to look at repair/replacement to resolve this.

You haven't answered my earlier question is it all networks, or just your Home network?