Yes they are done if you think otherwise then you don't know ND. The real question is.. is Sony done with it ? let's hope so because i don't want any other developer touching this franchise.

So yes i think Uncharted is done for good.

Umm no.. when are you guys going to realize that a new Jak game is not happening.. ever  

I hope that Naughty Dog is done with Uncharted.
Uncharted 4 was a great game to end the franchise and it is always the best to end a series in the peak.
If there will be a new game with Cassie it shouldn't be called Uncharted imo.

And don't forget that even if Uncharted is very big, Sony allowed then to kill Crash and Jak in the past and also freed them to do TLOU. So I don't see Sony forcing them to do UC, but perhaps they could make RaD develop a spin off.

If they do it will be Sam and Sullys story.

If they do make another uncharted it will probably be in about 10 years or more and most probably will be Cassie playing the lead with Drake as the Sullivan character.

Just for the humour though, I'll say they will release Uncharted: The Taken Treasure. Cassie is kidnapped during an expedition by Rafe's son who is running an illegal human trafficking ring in south America. Drake has to come out of retirement and put all his skills to use get her back.

Could work, but I rather not go there

Unlike the majority of people, I do actually want to see another, I'm intrigued to see Cassie as a main character, maybe 7 years on from uncharted 4 and Cassie is 20 years old, I don't know how the story would go but the Naughty Dogs writers are obviously imaginative enough to easily come up with something that will just suck everybody in again. People were saying Uncharted 3 should be the last but then they came up with this gem of a game. I also do think Cassie has been introduced for a reason.

I think Naughty Dog is done with UC but Sony probably isn't.

Naughty Dog can do a new IP if they want.
And it would probably still be great.