Hello, autosize is a fetaure that should be used to ensure that your volumes never go offline due to filling up. It automatically increases the size of the volume based upon some defined almost full thresholds, and allows you to sleep at night without having to manually monitor the status of all your volumes.
In your case I believe it should be used to compliment rather than replace your current working practice. I would enable autosize on the 4TB volumes, use them as normal and creating a new datastore when required. Autosize will then allow the previous datastores to grow naturally over time and expand as necessary.
You will need to monitor the autosize events to ensure the maximum size setting is not reached, otherwise next time the volume will not automatically increase. There is an EMS event message called wafl.vol.autoSize.done that should be monitored in OnCommand Unified Manager.
Hope this helps.