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Can you help me with my software licensing question?
We are thinking about getting the VMware essentials plus kit for 3 hosts, but in far future we would like to manage 4 hosts. Is there some cheaper way how to extend the license ? Or we have to buy the vCenter Standard with vSphere licenses ?
The vCenter for Essentials only supports 3 Hosts so no you can't add a 4th host to the vCenter for Essentials, worse still is that the 3 host licences you get with it can't work with a regular vCenter (Standard) later either - so you'd really limited if you go this way. I'd strongly advise going for one of the Standard or Advanced Acceleration Kits, they're more expensive today but don't limit you in the future.
Do you need the features that come with the Plus kit? You state in your question that you're thinking about it, which tells me that you haven't actually performed a needs analysis and are not sure what you need. After I perform a needs analysis I know what I need, I don't have to think about it.
The features available via Essentials Plus as opposed to Essentials are:
Data Recovery
High Availability
vMotion
With 2 of the 3 requiring shared storage (HA and vMotion).
So if you're not planning to implement shared storage and utilize HA or vMotion, my recommendation would be to purchase the Essentials Kit rather than the Essentials Plus Kit.