Java driver 4.x does not support advanced shard awareness afaik, meaning that it won’t use port 19042. Shard awareness will still work - the driver may just take longer to connect as it will have to build connection pool the traditional way, by opening connections to “random” shards.
See my post here: Questions about scylla java driver 4.x ShardAware feature and NAT issue - #2 by Lorak about my explanation of “shard awareness” and “advanced shard awareness” terms.