If Executive 1 Foxtrot reported the C-17 in sight, then the TRACON controller would have cleared them for the visual approach with the instructions to follow the C-17. At this point, the pilot is then providing their own separation. If the controller mentioned that they were at 4 miles separation on the hand-off in this situation, it was likely as a heads-up to the tower controllers.
If, however, the pilot never reported the C-17 in sight, then it's a different story.