Generally, Bronx street boxes increase in number from south to north, beginning with 2103 in Mott Haven. Between 2500 and 4099, if the first two digits are odd (i.e., 25xx, 37xx) the location is west of the Bronx River. If the first two digits are even numbers (28xx, 36xx) the location is east of the Bronx River.
Boxes 4100 through 4565 are in the northeast section of the borough. 42xx are mainly in the Throggs Neck, 43xx"s in Pelham Bay/Gardens, 44xx CO-OP City and Bay Chester, 45xx just on City Island.
This was all laid out in the early 1920s. Prior to that, all Bronx Box numbers began with a 2- to differentiate them from Manhattan boxes. As the Bronx population exploded, circuits 46xx, 47xx, 48xx, and 49xx were overlaid on the existing circuits for better street coverage. These also conformed to the east/west of the Bronx River. Examples: 4600 in Parkchester, 4800 on Pelham Pkwy; 4723 at the Richmond housing complex, 4902 in Bedford Park.
There are no 50xx boxes. Above that are basically Class 3 boxes (51xxs the Hunts Point Market), subways, roads and schools.
Hope this helps