The First digit represents the Borough.
All local routes in Brooklyn begin with 4.
In Staten Island, all local routes begin with 6.
In Queens, all local routes begin with 5.
In the Bronx, all locals begin with the number 3.
In Manhattan, local routes from M1 - M99 begin with 1 and routes M100 - M116 and up begin with 2.
Express Routes begin with the number 7.
A few routes that have a lot of destination signs start with the number 8.
Subway Shuttle Signs start with the number 9.

4   3  0 1

The second and third digits represent the Route.

4   3  0  1

The last digit represents the sign and the direction. Even numbers are for Northbound and Eastbound routes. Odd numbers are for Southbound and Westbound routes.