There are so many variations on Tier 3 storage that it’s hard to categorize this entry. Tier 3 is essentially data at rest, but data that might need to be accessed in the future. Tier 4 storage is a storage system that can still deliver that data back to you in short order. The common requirement in data indexing will be the ability to access that data. Having that data stored in a simple open interface file system like CIFS or NFS will be critical next we will finish up our “Tour of Tiers” with Tier 4.”]
Source: https://www.darkreading.com/database-security/oh-tier-3-