You show a solid body that has 8 faces, not 4 faces, since it has the topology of a U-channel.
Have you examined this geometry in SpaceClaim on the Repair tab and removed extra edges and extra points?
If you want a clean and uniform hex mesh, slice that solid body up into 5 bodies: the two large faces, the side face and the two long corners. Then use the Share button on the Workbench tab.