4 flush writers isn't bad since the default is 2. It doesn't make a difference if you have fast disks (like NVMe SSDs) because only 1 thread gets used.
But if flushes are slow, the work gets distributed to 4 flush writers so you end up with smaller flush sizes although it's difficult to tell how tiny the SSTables would be without analysing the logs and overall performance of your cluster. Was there a specific reason you decided to bump it up to 4? I'm just trying to get a sense of why you did it since it might provide some clues. Out of curiosity, what do you have set for the following? - max heap size - memtable_heap_space_in_mb - memtable_offheap_space_in_mb >
