There is a known limitation in the Windows 32bit Operating Systems which can't handle drives larger than 2 Terabytes (GPT volume). This results in volumes larger than 2TB not being visible on the disk management screen.
To overcome this limitation with a DriveStation Quattro 4TB, please divide the volume into partitions of less than 2TB (2048 GB/MBR volume).
Any actual Buffalo-NAS (SMB storage) device like LinkStation/TeraStation will have the same limitation, as it won't be able to handle volumes above 2TB on a USB connected storage device, there is no solution for this limitation.
To overcome this limitation with a TeraStation Pro II iSCSI 4TB, please use the LVM (LogicalVolumeManagement) function on the web-gui, to create volumes below 2TB (2047GB max.).
After connecting these volumes to your Windows XP, you can use the Windows Disk Management, to create Dynamic Disks (right click "Disk*" and select "Convert to Dynamic Disk").
Those disks can be spanned together (right click "Disk*" and select "New Volume" and "Spanned", "ADD" booth volumes and finish the setup) and you can use the full amount of your volume.
These Dynamic disks can be even connected and used with other PC's and OS, you just have to connect all volumes of this device and then you need to import the disk settings (Windows Disk Management -> right click "Disk*" and select "Import Foreign Disks").