I bought my 3 Radion XR30s at the time of the switch over from Gen3 to Gen 4 so the Gen3 Pros and Gen4 were the same price. I got 2 Gen3 Pros and 1 Gen4.
I've got the Gen3 Pros on either end of a 5' tank with the Gen4 in the middle.
Using Ecosmart to program the lights, the spectrum choices look the same.
Doing PAR measurements, all the lights seem to produce the same level of PAR at different depths but I haven't mapped out a precise grid like BRS does.
My tank is a mixed reef. Most LPS are euphyllia and M. lordhowensis on the sand bed at each end under the G3 pros.
SPS consist of Montipora stellata on the left, purple Stylophora center under the G4,
seriatapora hystrix, various millipora, montipora confusa and mystic sunset montipora on the right.
I don't see any difference in growth rate or health of the SPS under the different lights.
The Stylopora under the G4 grows like a weed and is consuming it's neighboring pocillapora and M. digitata.
I have notice that at any given power setting the Gen4 is using 15% less watts then the Gen3 pros do.