If you decide to upgrade your cluster, you should download 4.0.1 first(base image for 4.0.x) and then download and install 4.0.5. I wouldn't recommend just upgrading to 4.0.4 as there are bug fixes deployed for this panos version in 4.0.5. Here's the guide that will guide your upgrade process from 3.1.x to 4.0.x in a clustered environment.
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