This isn't exactly the same situation as the Halos where some were smaller and on the original xbox. The Uncharted games and pretty big, especially 3 which I hear is massive (not sure how big though). So the question is do you think ND can compress all onto one disc?

Each one took up 25GB, and the latest Blu Rays hold 67GB double-layered, and 128GB quad-drouple layered.  I think they easily could.

No. It would be a two disc set. 1-2, 3(4). Especially if they put in higer res textures for 1080p 1-3.

Can't the PS4 handle those tri- and quad-layer blu-rays discs? Sony could use one of them if they could.

We don't know if the PS4 can play BDs with more than 2 layers though. And I don't know if 67GB double-layered discs actually exist. There being 100GB triple-layered discs, of course tells that it should be possible to fit 67GB onto 2 layers. But 67GB will not be enough anyways. UC3 was released on a dual-layer BD and alone has a size of ~45GB. Add UC1 and UC2 and some higher res textures and at least 100GB will be required.

And discs with more than 2 layers are way more expansive, even if they work in the PS4. I'm not saying that we will never see them, but I think that it's way more likely they just put 2 discs inside the box (with UC1 + UC2 on disc 1 and UC3 on disc 2), if we get an Uncharted collection. 

I thought BR discs, at least for games had a capasity of 50gb...

Disk 1: U1 + U2
Disk 2: U3 + UGA + UFFF
Disk 3: U4

Not gonna happen though...

I don't think U4 will be in the collection, at least not for now.

they would take up more with the DLC, up res, better models, and other improvements.

It's possible with good audio compression.