Hello @hwm.land,
Welcome to the forum!
I suppose you mean 4TB disk.
This is mean that either you disk is dismounted, or it doesn’t have a partitions table and you formatted the disk without creating a partition.
I would suggest to check that the disk is still mounted:
df --si -T
If it’s mounted, but empty, I would suggest to repartition it, as described in the guide:
Or use an LVM to create a logical volume, then format it with ext4.
You may see an example there:
It’s also possible that you just need to check it and fix errors with fsck.