It has been revealed in an interview with Uncharted 4’s Creative Director, Neil Druckmann that Uncharted 4 will be the longest in the series. This is one of the main reasons that the game was delayed from 2015. The team considered cutting sections out of the game however decided that the Uncharted 4 story would benefit from having a longer story that was more fleshed out.
You can find the full quote from Neil Druckmann below.
“When we realised we weren’t going to hit 2015, it was sequences that were just coming online. There’s an option on the table to cut that sequence out and see if you can stitch the game together without it, but you risk hurting pacing in the story when you do that. Everything we put in there felt necessary to tell the story. It’s very ambitious, it’s the longest Uncharted in the series, and the most varied in terms of location.”