Recently i tried to upgrade my cassandra cluster (3 + 3) from cassandra 3.0.15 to 4.1.0 in rolling upgrade manner, and i figured out that when a node with older version of cassandra act as coordinator, it cant read data from newer version of cassandra and vice versa. so there is a huge inconsistency happening in the middle of the upgradation process. and i assume its happening because of schema mismatch between C3 and C4.