My iPhone 4s won't auto connect to Wi-Fi?

So I keep connecting my phone to the Wi-Fi and when I end up using it it will just disconnect it won't auto turn on like my old iPhone 4 did why? Could it be iPhone 4 do it but 4s doesn't cause I had my iPhone 4 with Verizon and my 4s with AT&T now please help me.

You have to activate your "personal hotspot" on your phone and turn off your phones internet. Once that is done, turn on your Wi-Fi and your phone will search for your router.

You might not have put to auto connect to that specific Wi-Fi. Delete the Wi-Fi connection on your phone then try logging into the Wi-Fi again.

I used to have this problem with my 1G and 3GS. It would never connect or auto detect Wi-Fi after you save the password. It used to work only on Data.
If you have dropped it down one-too many times then that might be it.
Or it could be moisture contact.

Take it to the store if you are sure its not your fault and have it replaced.