There are round 2x2 pieces. There are round 4x4 pieces with round 2x2 holes, the 2x2 can slide/rotate inside of it. There are "macaroni" pieces to make an 8x8 circle with a 6x6 hole. But are there no 6x6 rounds with a 4x4 hole?
I've also seen sets use curved slopes, curved inverted slopes, and arches to make circles or nearly-perfect-circles. Are there ways to make nested circles from those pieces?