I think that organ builders are pretty much divided on the relationship
of the 8 and 4 foot principals, with some thinking that the 8 should be
voiced bigger and others that the 4 should.
 
Personally, I like the 8 foot to have a well developed harmonic series so
that it can sing.  I remember one particular organ (the large Aeolian
-Skinner at Northwestern) where the eight foot has a large scale, but
little harmonic content.  The builders idea was that the 8 would rarely
be used alone, and that it needed to be big to support that plenum.  The
four foot was well developed harmonically and there is no doubt to my
ears that the plenum worked well.  On the other hand, I really didn't
like the dullness of the 8.  I also found that there was a big jump in
timbre between the 8 alone (very dull) and the 8 and 4 (very bright).  I
didn't care for this.
 
A few years ago I discussed this with an organ builder whose instruments
I really respect.  I won't name him, for fear that I am
misquoting, but, as I recall, he said that his 8 foot principals needed
to be warm *and* harmonically developed so that they could be used alone
(and indeed, on his organs, the Principal 8 is a lovely solo stop), and
that he believed that the 4 was probably the most utilitarian stop on the
instrument: a little uninteresting by itself, and totally a "slave" to
developing the chorus.  I personally *really* like the way that his
plenums build.  You can use 8 or 8 & 4, or 8,4,2 etc. (even 8,4,2 2/3
works wonderfully).
 
So I guess it's not just an issue of scaling or voicing, but both.  I
vote in favor of large, harmonically devloped, warm, singing Principals
with a subservient 4 foot.
 
Joe Downing