NexentaStor 4.x has finally made it to GA as of today. I will be evaluating if it makes sense to make the move yet or not prior to upgrading my actual production lab array. Keep in mind that this is a complete reinstall so the upgrade is disruptive. They claim the next release will allow an upgrade from 3.x to 4.x non-disruptive. I am curious about that one :)
In the meantime here is a link to the NexentaStor 4.0.1 release notes. I advise you to take a look.