Unfortunately, there is no way to take advantage of that unused capacity with your current configuration. The only way to access unused array space is if you can expand the volume to fill the entire array or can create a second volume that uses the remaining capacity (both of which would require 4 disks, all with space). In order to create a second volume on an existing array, there needs to be space across all the disks on that array; you have a 4-disk RAID 0 volume, so any second volume on that array would need to also use all 4 disks. In order to expand the volume, there needs to be space on all of the disks in that array to expand into (e.g. max volume size is always limited by the size of the smallest disk in the array).