Great post.
Based on your observations, it would seem (my guess) that the connector you used (not the connector quality) with your 4S is not bypassing the iphone's dac.
I found this on Crutchfield's website.
iPod/iPhone Control
iPod Connectivity w/ DAC Bypass: The Pioneer AVH-4000NEX supports direct connection of your 30-pin or Lightning-connector iPod/iPhone through its rear-panel USB port, HDMI input, RGB port, and 3.5mm jack. The AVH-4000NEX offers audio/video playback, music navigation, information display, and charging of select Apple iPod/iPhone models. It also provides a sp