I've also enjoyed what I've played of Uncharted 4. The gameplay is further refined, making it the best in the series. I really like how the combat sections are more spread out. The last chapter I played, Chapter 12, was entirely exploration and a few puzzles.
Naughty Dog addressed my biggest concern with U3, which is that it was too easy. I'm playing on Crushing, and this game is much harder. Not as hard as the first game was at its most difficult, but more difficult than 2 and 3.
My biggest complaint so far is that there has yet to be a massive action sequence on the level of 2's train section or 3's cruise ship, but that will hopefully be addressed in the game's final third. Everything else, from story to gameplay, has been on point. I look forward to finishing it tomorrow.