The Pro4 is an evolution of the Pro2 EVO rear hub which it replaced. 

The Pro2 used a 15mm axle (and therefore bearings with a 15mm inside diameter) which was fine in the quick-release days but axles started breaking once 12mm thru-axles became the norm. The Pro2 EVO uses bearings with a 17mm inside diameter in both the hub shell and freehub body.

The Pro4 hub which superseded the Pro2 EVO shares the same axle and endcaps but a different freehub body.