Good question, I am not the one to search the limits and like to replace the rod after a 5~7 liter use. So far I have no issues with the R4 rod replacing it at the same time as I do with the R7

Here you can see the differences:
R/C Tech Forums - View Single Post - Novarossi Ielasi Tuned Engines - Official Thread

And yes, lightweigh should mean more performance but placing a R7 rod I did not notice a thing.