@GPeeps4 

Yes, you can leave your AT&T device alone if you want. Connect the S4 and set it up normally. What you will have is the S4 wireless network, and the AT&T network. What would be recommended is to turn off the Wi-Fi of the AT&T, and set the S4 to use the same wireless network name and password. That way your existing wireless devices will automatically connect to the S4.

@Tony thank you for the reply!  So do i hook up the S4 using one of the internet ports on the ATT modem/router or plug it into the Ethernet wall outlet for setup?  Not sure how to "turn off" wifi on the ATT box.  This is not my gig, so pardon any ignorant questions!!

@GPeeps4 

You would plug the S4 to one of the LAN ports of the modem/router so say if you had a computer that needed internet, whichever port you would plug that into.

If you are not familiar with being able to turn off the Wi-Fi, you could leave that alone and set up the S4 with a different network name if you do not mind connecting your wireless devices to the new network.

@GPeeps4 So your connecting the S4 as a router not an access point, Correct?