You’re missing a point: if R4’s run on a separated class from R5’s, they don’t need to be as fast as those! Guys with a lower budget will prefer to fight for a R4 class win than struggling to stand up in the middle of 15 or 20 R5’s.

You should look here http://www.rmcmotorsport.es/ to see how the Spanish tuner RMC is promoting N5 (similar to R4) in Spain. N5 is based on Argentina’s Maxi Rally (actually RMC imports a large number of parts from Baratec) and despite being slower than a R5 and having only national homologation it’s selling pretty well, once they’re cheaper and run on a separated class from R5’s; besides RMC is doing a proper job promoting them (there’s even a national N5 Cup).

Everybody recognizes that there’s room for a lower 4wd class and it’d be much more interesting to see it spreading as an international FIA category, in R4 form, than having each country trying their own national proto class, like it’s now starting to happen.