I agree with gunslinger, plus:
There was a '63 Avanti with a crate R4 engine near me back in the late 1960s. I followed the car through 2 or 3 owners, probably NONE of which understood the implications of a 12:1 C.R.; i.e., necessity for using racing gas. Who is going to do that in a street-driven car? Having 12:1 compression ratio on a street-driven car is a very bad idea. Plus, the R4 is very over carbureted. The cam would have been either 276 or 288 deg. same as an R3. An R3, with its 9.75 C.R., would have been a much better choice than an R4. Simply remove the carb enclosure and install an R1 air cleaner. (also dist. advance, carb jets, etc would need to be revised, but those aren't big deals)
BTW, the R4 Avanti locally ended up with two badly damaged cylinders, and most if not all broken rings.
Perhaps R3 engines WERE available and an R4 wasn't. There were a hundred or so R3 engines built, but only something like 10 R4 engines.
Note that R3s and R4s used the same cylinder heads (479). The difference in the comp. ratio was in the pistons, not the heads.