The Device Support Matrix for DP 8
http://support.openview.hp.com/selfsolve/document/KM00453104 pg 30
shows StoreOnce support only for a B6200 StoreOnce Media Agent. It may work with a B4200, but that is not a certainty
Configuration instructions for a StoreOnce Media Agent may be viewed in the document
I did not find a Guide specifically for DP 8, but, I feel pretty sure that this has not changed radically from what is was in DP 7
Other members of the Forum may have real-world experience with integration with B4200