This is the description for aero j-style pickups: Type 1, Std (Neck/Bridge) - Reviewed in the March 1999 edition of Bass Player Magazine, Aero's J Bass 4 String pickup won a "Top Tone" award. The complete comment, "Heavy lows, distinct upper mids, and round top that's cool for fingerstyle rock, blues, and country. (And '70s-style funk.) Broken-in vibe that's fun to play. On tape, pickups have big bottom for a J, although the tone can get a bit thick." Type 4A - Type 1 pickup with four 1/4"OD pole pieces. The pickup has a narrower field than the Type 4, but has exposed pole pieces. Ok, what does "narrower field" mean exactly???
It refers to the range of the string that is actually "read" by the pickup. For example, a pickup that only read 1/4" of string length would be considered to have a far narrower field than one that picked up 3/4". Another way to easily visualize is that humbuckers have a much wider field than single coils; think MusicMan compared to a Jazz Bass.
FWIW: I'm guessing maybe you wanted more of a difference in tone interpretation than literal. DavidRavenMoon designs pups so if that was what you were asking, you might drop him a PM/email.