If you already have 4 application servers you are probably already AZ tolerant; most people concerned about "doubling everything" are only running 1 instance.
Going by your example, If your website requires 1 application server, to tolerate a single AZ failure, it requires you to double the number of application servers.
Example - we have a service that used Kafka in the affected region that went down. Our primary kafka instance (R=3) survived but this auxiliary one failed and caused downtime. There's no way around this other than doubling the cost.
