Assuming you want to clone N003:
Step 1: copy the entire "N003" folder (ctrl+c, ctrl+v).
Step 2: rename "copy of N003" to "N004"
Step 3: rename EVERY FILE beginning with N003 to N004.
The easy way to do this is:
1) open a command window (Start - run - cmd)
2) go to the folder (cd "My Documents\EA Games\The Sims 2\Neighborhoods\N004", enter)
3) type this command: REN N003* N004*
4) go to the first subfolder (cd characters, enter)
5) repeat 3
6) go to the second subfolder (cd .. , cd lots, enter)
7) repeat 3
go to the third subfolder (cd .. , cd Storytelling, enter)
9) repeat 3
10) go to the last subfolder (cd .. , cd Thumbnails, enter)
11) repeat 3
Done.