I looked into this a while back - ultimately I decided to get a Garmin 810, but I think you could put together a decent system with an S4. Here are some thoughts:
Mount: Take a look at this one:
GS4-PP ~ BikeConsole Power Plus for Samsung Galaxy S4
It's a mount and additional battery all in one.
App: The S4 can natively use Bluetooth and ANT+ sensors, so you've got a lot of options. I would try Endomondo, don't know if you'd need the pro version or not. There are other options like IpBike, but Endomondo is pretty slick.
Sensors: Any ANT+ or Bluetooth sensor will work fine with the S4. Just make sure the app you use supports the sensors you have. There are some apps that only support Bluetooth sensors. The new Garmin accelerometer-based speed and cadence sensors look really good and are starting to ship:
Garmin - Bike Speed & Cadence Sensor