That's two pretty big figures of Naughty Dog gone, all within a rather small time-frame at that. Justin Richmond was the main man behind Uncharted 3, originally the director behind Uncharted 4, and was especially entrenched on the multiplayer side of things for both Uncharted 2 & 3. Guy's now working on League of Legends at Riot Games, so I suppose multiplayer developments appear to be his preference.
Anywhoo Sony have of course said that his departure will have no impact on the development of PS4 Uncharted ect., as you'd expect. Kinda puts this new Uncharted in a bit of murky light to lose two key members of the team like this. But then, hey, I think Uncharted 3's single-player was heavily flawed, so... perhaps it isn't quite a worst case scenario for the next Uncharted to lose one of the head honchos from aforementioned heavily flawed Uncharted. Though the loss of Amy still continues to suck.