I've read it many times where Vinnie quoted the 5.5g iMod as having better bass. The iPod and iMod line out have a big difference to them in sound, they really aren't very close.
found it after a long time searching:
New iMod for the iPod 5G: A Red Wine Audio/ALO Collaboration
Its not a question for how close is the 4th gen with vCap dock is to a 5th gen iPod. No iPod can get enhance quality with the vCap dock without being iModded and you can't use a vCap dock with the 4th gen iMod as only the headphone jack is modified.