Well ive been doing some measuring up today and the front will fit but the discs are 1mm further appart on a gen4.
The rear does have different offsets and i though id just stick the gen4 carrier on the gen3 rim but although it fits ok the offset is 2mm different on the carrier.
The wheel offset on the carrier side is 2mm different aswel so if you run gen4 carrier on gen3 rim it will be 4mm out of alignment which is a bummer as it would solve the bearing diameter if i were to convert all to 25mm spindle.
So you ideally need a gen3 rim and carrier to keep the left offset the same.
The right offset is different too as the brake disc doesnt sit in the same place.
I cant remember the measurement off the top of my head but im sure it could be got around.
So to me i need to have them fully assembled and weigh them with the spindles in as they dont seem like they are much heavier if at all.
Ideally i would have liked both rims being made the same diameter to make wheel swaps easier.
Below is a pic showing 2mm difference in height from left gen3 carrier and right gen4.
Height of bottom of sprockets is different and the bolts are sticking up higher on the gen4.
All measurements are carried out using vernias but not 100% as ideally i need to use my height guage at work for perfect measurements.