There will be an effect upon the string from two magnetic fields as opposed to one if you have a second pickup - whether I could hear the difference in a blind test I've no idea!
I also lack the expertise to work out whether the extra volume pot and adjoining wire would give an audible difference.
And then we're back on the debate as to how to tell without physically removing the bridge unit from a IV and recording the instrument - as two basses can sound quite different even when they are identically spec'd?
I've always preferred P to J basses, as the rear p'ups rarely enthuse me (when I'm playing them, that is) whereas I love twin bucker p'up basses! So I can def get alongside the notion of IIs sounding great bcoz of my affinity with both sorts of P (I haven't had the opportunity to play a II yet) but I absolutely love my IVs for tone! And happily turn either pickup all the way off for various tunes/tones.
