Aside: We have Scylla Cloud. It runs on AWS. Our next major release includes LWT and CDC, so any major gaps will be closed soon. And yes, we're looking at Cassandra 4.0 closely.
In some ways, we will implement features similarly to Cassandra but subtly and importantly different. Example: We do both local and global secondary indexes (Cassandra only has local). Also, our CQL LIKE function is closer in implementation to SQL LIKE than to Cassandra's. And so on.
btw: ScyllaDB just got $25M in new funding in September 2019. We're doing fine financially.
It started like gangbusters, but I get the feeling the dev money ran out because the feature rollout has completely stalled.