There's no such thing as off-map lots in TS4.
Well they don't appear in the UI but the team still needs to make a surrounding neighborhood for them. Magnolia Promenade and the career lots aren't recycled maps. They just used the world props of the already made worlds. GTW came with 4 neighborhoods technically but I guess making the actual ui maps for those can come out as expensive according to a simguru.
Honestly I think a new world should be made if it makes sense and/or if the budget allows for it.
In packs like Parenthood a new world doesn't make any sense because they are just extending gameplay related to the sims themselves. They didn't add any new significant venues or new active careers nor it revolves solely around a world (like CL).
