135s are just fine for the small engined R4s, it's always beneficial to cut that little rolling resistance.
BUT...two years ago I discovered that "big" tyre makes stopped making them, so I had two options: Switch to 145s (still plenty of choices) or fit a set of Korean or Eastern Europe 135s. There is nothing wrong with less known makes of tyres, especially on the R4 that relies less on tyres for its roadholding, but I have found them to be generally heavier than an equivalent "known make" tyre.
So I fitted a set of 145s Continental Eco Contact (210 euros including inner tubes). I am very pleased with them, and going one size up doesn't seem to make noticeable difference. I noted that even on the parts books, fitting of 135s or 145s is equally allowed on saloons (maybe it was the buyer's choose?), while on Fourgons, 145 was the standard fitment.
Just to compare prices, two weeks ago I had a set of 145s BF Goodrich fitted to another R4, for 180 euros.