The 4 express idea won't work since that will skip too many important local stops (i.e 161, Fordham, Bedford Park)
There is no point in having the Lefferts A skip only 2 stops. Plus trains are stored on the middle track during the middays.
The 7 is not all THAT long.
If people wanted a "quicker" ride to upper Manhattan/ Riverdale, they could just take the A to 168 and change to the 1, or even better, ride Metro North.
All because there are middle tracks throughout the system does not mean it is practical to run express service.