So this makes Djokovic the leading member of the Big 4.
Well Nole's days in the top 4 are numbered.

We have just left traditionally the strongest part of Djokovic's season (save the two Masters 1000s in the Fall.) He's only sixth in the race with Zverev ahead of him.
Zverev will likely expand that lead on grass. After some resurgence at RG Murray close behind at 7 seems poised to pass Nole too for good. Dimitrov is 400 points behind, but may close that gap by the end of Wimbledon. Djokovic is lucky Goffin is out for a while on grass (returns Wimbledon.) Also lurking our Raonic, Kyrgios, and Tsonga (3 titles already this year). Djokovic still may not qualify for WTF.
Before WTF failure happens Djokovic must drop out of the top 4 in the rankings at some point.

If Federer runs the tables on grass it could happen with his Wimbledon triumph. Otherwise, it's likely to be a nice early 36th birthday present for Roger after Nole's Rogers Cup victory drops.