I simply flew 10 hours solo, so then I on my application I could say I had 10 hours in an RV4, and there was no further discussion.
Right before flying it for the first time I did landings in 3 very different tail draggers (decathlon, J3 cub, great lakes biplane), with an instructor because that was the only way to rent those planes, but instructors are good. I figured if I could land those three just fine then I could land the -4 just fine also.