Community:When to keep a parallel 3.x instance
From Splunk Wiki
If you have a significantly older 3.x instance (3.2.x or earlier) you could be better off continuing to run it in parallel with a new 4.x instance.
You may particularly want to do this if your older data relies on features changed or replaced by 4.x such as transforms, metaevents or custom search commands and scripts. Also if you have a modest retention policy that would age out this data soon anyway.
If your old instance only contains historical data and is not indexing new data, there are no license considerations for this kind of deployment.