With acks=all, the leader waits formin.insync.replicasto acknowledge the message. Since min.insync.replicas=3, Kafka will only commit the messageonce 3 brokers (leader + 2 followers)confirm they have the message.

FromKafka Documentation > Acks and Durability:

“If acks=all is specified, the producer will wait until the full set of in-sync replicas has acknowledged the record. The minimum number of in-sync replicas is controlled by min.insync.replicas.”

Even though the replication factor is 4, only3 acknowledgments are needed, as defined by min.insync.replicas.