I can see 2 problems here:
First possible problem:
The nas comes with 4 disks of 1 TB configured with RAID10 (or any other drive RAID combination that gives you 2 TB total storage) and isn't able to resize the RAID array size when larger disks are added run, don't walk away from the product - then there's nothing you can do about it but a host based migration
- copy files away from the NAS
- replace all disks
- completeley re-initiate the RAID with the new drives
Second possible problem:
The device is perfectly capable from a feature point of view but has only certain disks supported, the largest supported disks available yield a config of a max. of 2 TB only. "Supported" in this context means disks listed on the compatibility list of the vendor, not actually supported from a technical point of view.
Marketing speech. Often marketing will release a product with a folder which says "Up to n TB of storage supported" most times this refers only to the maximum disk size available at the time of the writing, but the device doesn't have a:
- missing feature preventing you from actually migrating RAID levels
- a hardware compatibility list preventing you from using larger disks (in terms of accepted support cases)
Can you update your question with the actual error message or are you right now just investigating options and the limits are from a product fact sheet?