So I was on the phone with my son (Bebo), when i get off the shuttle from Prospect i see a 4 pull up saying:
(4) Franklin Av
(4) via 7 Av Lcl
(4) Eastern Pkwy Exp
and the 5 was:
(5) Crown Hts - Utica
(5) via 7 Av Exp
(5) Eastern Pkwy Lcl
It appears that a rail condition at 77th street on the (6) forced them to run it on the express track. So they ran the (4)(5) via 7 Av and the (6) exp between Grand Central and 125 in both directions.
I wish i had some pics but it shows the MTA can make some crazy re-routes.