Well, a stock apheared 47 amp would not be too good for the 4s, which has quite a high impedence. And the main design feature of the47 amp is the paralleled opamps, which is unnecessary for high impedence headphones. But more current is always better, and the 47 will sound *slightly* better with any headphone, as long as the gain is set high enough.
So the modifications to the basic 47 amp would be to increase the gain. Another think i really think should be done is to add an input buffer to isolate the x-feed, but that can only be done on a custom design, not a stock CHA board. Also, i really like apheared's new HD600 design, should be GREAT for the ety 4s (also works better with x-feed than a 47 amp) but it needs to be custom built on a blank pcb.