Reddit reviews Hyperion Dual Desktop Charger Cradle / Dual Dock Charger For Samsung Galaxy S4 With Battery Charging Slot Includes Hyperion Samsung Galaxy S4 2600mAh Spare Battery (Compatible with All Samsung Galaxy S4 / SIV / S 4 / S IV Models) 18 Month Warranty

Then I think you're leaning the right way. The GS4 lasts all day with moderate use from most reviews I've read. The SD card slot is a must for stock 16GB version, but I don't know how much the GE OS will take up on the hard drive.
If you're looking for expanded batteries, then I'd suggest these that were recommended by /u/moar_distractions:

• Zerolemon 7500mAh battery ($40)
  http://www.amazon.com/gp/product/B00CXZI3DQ/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1
  
• Slightly more manageable 5200mAh ($25)
  http://www.amazon.com/Hyperion-Samsung-Extended-Compatible-Warranty/dp/B00BCK2G40/ref=sr_1_6?s=electronics&ie=UTF8&qid=1371040849&sr=1-6&keywords=galaxy+s4+battery
  
• A case for the Hyperion 5200 battery that looks pretty sick http://www.amazon.com/Hyperion-Samsung-Extended-HoneyComb-Packaging/dp/B00CXO09DY/ref=pd_sim_e_2
  
• Set of two standard size, supports NFC and Wallet, comes with a charger, looks promising to me ($25) http://www.amazon.com/Hyperion-Samsung-Batteries-Compatible-Warranty/dp/B00BCJWYMU/ref=sr_1_8?s=electronics&ie=UTF8&qid=1371040849&sr=1-8&keywords=galaxy+s4+battery
  
• 1 Hyperion standard size, with docking cradle that charges the extra battery (what sized cases fit?) ($30) http://www.amazon.com/Hyperion-Charging-Includes-Compatible-Warranty/dp/B00CP8EK1U/ref=pd_sim_e_3

quick search on amazon -

http://amzn.com/B00CP8EK1U

http://amzn.com/B00C4CVSSU

http://amzn.com/B00D3I6S4I

http://amzn.com/B00C4CVT02

http://amzn.com/B00CMNJ6LM

I can think of times this would be helpful, long road trips where everyone might be fighting over one charger, long flights, camping. I just can't see myself having it permanently on my phone. Here are some other options I found quick on Amazon just in case people are interested. For me personally, I think I'm going to get a 2 pack of standard batteries that come with their own external charger. I can keep those in my backpack (or one in my pocket) and between the regular one and backup I can't see myself needing more.

• Slightly more manageable 5200mAh ($25)
  http://www.amazon.com/Hyperion-Samsung-Extended-Compatible-Warranty/dp/B00BCK2G40/ref=sr_1_6?s=electronics&ie=UTF8&qid=1371040849&sr=1-6&keywords=galaxy+s4+battery
  
• A case for the Hyperion 5200 battery that looks pretty sick
  http://www.amazon.com/Hyperion-Samsung-Extended-HoneyComb-Packaging/dp/B00CXO09DY/ref=pd_sim_e_2
  
• Set of two standard size, supports NFC and Wallet, comes with a charger, looks promising to me ($25)
  http://www.amazon.com/Hyperion-Samsung-Batteries-Compatible-Warranty/dp/B00BCJWYMU/ref=sr_1_8?s=electronics&ie=UTF8&qid=1371040849&sr=1-8&keywords=galaxy+s4+battery
  
• 1 Hyperion standard size, with docking cradle that charges the extra battery (what sized cases fit?) ($30)
  http://www.amazon.com/Hyperion-Charging-Includes-Compatible-Warranty/dp/B00CP8EK1U/ref=pd_sim_e_3
  
  Does anyone have any experience with these Hyperion brands? They all come with an 18 month warranty which sounds pretty sweet.