After further testing, I find that there are good reasons for limiting iOS 7 to newer devices.
I tried playing 24bit/96kHz files via iTunes sharing on the iPhone 4/iOS7 through the microStreamer. The files play, and the 96k LED lights up, but there is stuttering. The stuttering does not show up on 16/44.1 files. Also, I have tested 48kHz files and they play cleanly at 48kHz on the microStreamer (48k LED lights up) using Apple's Music app. Not all players do this, however (for example, Equalizer by audioforge resamples to 44.1kHz even with 48kHz files).
The processor on the iPhone 4 is a single-core A4 at 800MHz. The processor on the iPhone 4s is a dual-core A5 at 800 MHz. The iPad 2 has a dual core A5 with a speed bump to 1GHz, and I have verified that the iPad 2 can do 96kHz playback through the CCK. So, hopefully the iPhone 4s will do 96kHz playback cleanly given its similarity to the iPad 2.
I may test the iPhone 4s at some point as we've got one in the family -- haven't done the OS upgrade on it.