Take a look here

There are some good responses to your question there.

Your link didn't work for me, krsx66. Anyway, I have a theory which I based on a few simple observations, but other than that it comes PURELY from my imagination.

Noob Saibot appeared in UMK3/MKT, he was being a spy for Shinnok. Now, that might be the reason Sub-Zero came back. Here are some reasons why I believe that:
MK2 and MK3 we see the portals and they are red, but in MK4, the portal is blue, which COULD symbolize the Netherealm.
We see that portal in UMK3/MKT, and although it's called "Lost" or something, I'm not sure where it came from but people call it Scizlac Burosez, which rearranges into Classic SubZero... Now, Here's the theory: Sub Zero found out that Shinnok is sending a spy, he somehow gained access to the portal and went to Earthrealm or Outworld, I dunno they were merging at the time. Anyway, he went there to warn his brother that something's up. Maybe that's when and how Unmasked Sub-Zero found out about the secret to kill Shinnok, (As we find out in in his bio in MK4.) It's an old theory that I've been developing, and it's not impossible. But Midway probably didn't think it THAT far through, they just wanted to Exploit the pallette swaps ot the fullest. That's my guess.

Sub_Zero_13 Wrote: Your link didn't work for me, krsx66.

Ah, you're right. Sorry 'bout that, I have no idea why the link isnt working, I've tried everything... If u wanna see the thread go tp P.7 and scroll down to "Who was UMK3 / MKT Classic Sub Zero supposed to be in MK4???" thats what I was linking too...