Tournaments and club nights often require quick reference. Having Book 4 stored on a tablet or laptop means you have the entire High Medieval army compendium in your bag without the weight of a rulebook.
The timeframe covered in Book 4 is staggering in its variety. It begins in the aftermath of Manzikert (1071) and the Norman expansion, concluding just before the massive shifts in military technology that defined the Italian Wars. Dbmm Army Lists Book 4 Pdf