USA Spector Owners, please explain...

Not a USA Spector owner, but I do (think) I know the answer. The Ns2 has the pj pickup configuration, and the NS 4 has soap bars. There may be other differences, but I think that's the key one.
If I remember right, during the SSD era Spector had to refer to their four strings as NS 4s because Ns2 was still trademarked to kramer. I'm sure someone more knowledgable than I can clarify/correct me.

 

You're kind of right...I think.

NS-2 and NS-4 are identical basses. Same pickups and everything. The name difference is, like you said, during the SSD era Stuart could not use the NS-2 name, so he named them NS-4.

Here is my old NS-4. Should have never sold it. Serial #84. Would love to have it back!

 

not an owner but i think ns2 is 9v and ns4 is 18v.

 

I think I see where the confusion is coming in...

Looks like we're all right!

Back when Stuart couldn't use the NS-2 name, he used the name NS-4. So, and SSD era NS-4 is the same as a Spector NS-2.

I guess when he got the name back, he kept the NS-4 name and gave it soapbars and 18v electronics. My NS-4 was 9v and had P/J pickups.

So..what is the OP asking about? An SSD NS-4, or a Spector NS-4?

 

Here's more info to confuse you further:
Scroll down to the NS-4, you'll see it's the same thing as an NS-2....but you could order it with soapbars and 18v electronics.
Stuart Spector Designs, LTD - Makers of fine electric and bass guitars: UNCOMPROMISING QUALITY | UNPARALLELED VERSATILITY | UNBELIEVABLE SOUND

 

i could be wrong, but i seem to recall one of the spector guys saying that originally, the NS-2 designated it as NS = Ned Steinberger and 2 = two pickups. when the ssd era came around the 4 denoted the number of strings. after Stuart got the rights back to use the NS-2 name, the 4 in the NS-4 designated the amount of coils the bass had (two soapbars - 4 coils)

 

Makes sense to me! Especially since my NS-5 has neither 5 pickups nor 5 coils

 

lol. ask jps. he could tell you. find him in the spector forums

 

In terms of tone the Spectors with EMG DCs are fatter sounding while retaining great clarity while the P/J setup is more cutting. The Reverse P pickup adds punch, don't expect P bass fullness down low but it's still pretty fat, dial in the P pickup for more bite.

The P/J setup is more the traditional Spector tone. I kind only slightly prefer the P/J setup.