I believe your due diligences has led you to two different concepts ...
"Is there a difference in sound moving from a 6' power cable to a 4' power cable"
Here I believe you are referring to the SWR (Standing Wave Ratio) concept .. where the length of the PC will set up standing waves or reflections
The least deliberating lengths to help minimize these reflections are 1.5 meters (5ft) and 2.5 meters (8.125 feet) ... at these length there will be the least amount of reflections to degrade the signal
"I read somewhere that you only hear that last 6' of cable"
Not quite .. I believe here you are referring to the argument about the importance that a power cable can play in your system ... being as it is the last 6 feet in the delivery chain
It is not the last 6 feet but the first 6 feet as it connects directly to your power supply and should be viewed as a part or extension of it
The two sides of the argument are the PC is the last 6 feet of the power deliver chain coming from the power company .. how can it improve the signal after all those miles of wire and contamination coming from the power company
Im from the school that says it "cant" improve the incoming signal .. but that last 6 feet can definately continue to add additional noise and distortion further degrading the signal coming from the power company
A higher quality PC will degrade the last 6 feet less and will sound better because it did not inject as much additional noise and distortion into the current vs the amount of noise and distortion injected by a lower quality cable
Shielding .. Geometry and cable length are the variables that can make a difference ... but you cant improve the incoming signal quality .. only degrade it further