The typical 4s 2200 30C battery weighs about a 1/3 more than a 3s battery and needs to be moved back to get the model to balance correctly on the step. The existing battery "channel," however, has a "wall" across the back preventing the battery from going back far enough. All you have to do is to cut out the wall, allowing the battery to protrude slightly into the space behind. This also gives you a place to tuck the connectors out of the way. It's a distinct improvement all round. You might want to add a small block of foam to extend the floor of the channel back by 1/2" or so, but it really isn't necessary.
I installed industrial strength Velcro (hook side) in the bottom of the channel and add a couple of patches of the fuzzy side of the Velcro to each battery. By the way, you'll find that the Velcro straps built into the fuselage get in the way. You may want to cut them off completely, but at the very least you need to shorten them, as otherwise they bunch up and prevent the cockpit unit from fitting properly.
As noted above, I'm very happy with the 4s setup for more performance but also consider 3s ample for normal flying.
If you do go 4s, be sure to:
- Use an ESC that is designed for 4s (many with linear BEC are only good for 3s and may shut down the power to the radio if overloaded)
- Change the prop to reduce the load (8x4 works well with stock motor)
- Get the balance right