Ok I did some checking today and it will fit on a V6 or V8.. I found that the factory exhaust system from the cat back is the same part number for the V6 and V8 and I read the TRD paper work that came with the system and it does not say V6 or V8 only...
I guess start out by giving TRD a call to see what they say...
Maybe someone on the forum could tell us if they have tried this,
I don't know anybody with a 04 V6 runner here in Cincy to see if they will work.
TRD Dual Cat Back Exhaust PTR06-89030-19 "Brand New" still in the box for 2003-2004 4-Runner V8,4.7 I bought this system to use on my FJ40 4.7 project but never did use it. $575.00 o.b.o.
Plus shipping.