Here's a quick look at how much NL managers have relied on their top 4 relievers (by appearances) vs. the rest of their bullpens, thru Sunday:
% of relief innings pitched by top 4 relievers, in reverse order:
Pirates 45% (87.2/184.2)
Phillies 48% (82.3/170.3)
Braves 50% (91/182)
Giants 51% (92.1/179)
Padres 52% (92/178)
Mets 53% (92/173)
Rockies 54% (113/209.1)
Nats 55% (97/186.1)
Cubs 57% (101/177)
Marlins 58% (93/161)
Dbacks 60% (100/166)
Reds 63% (111/175.1)
Factor in that one of the Reds top 4 is their closer, and you have an even more extreme pattern.