Worry not, this delay only pushes the game back by about a month, but it isn't the first time Naughty Dog has changed the release date on their game; Uncharted 4 was originally supposed to be released near the end of 2015, then was moved to March, and now April.
The new release date for Uncharted 4: A Thiefs End is April 26, 2016.
According to creative director and game director Neil Druckmann & Bruce Straley, the game has been delayed because there's still polishing to do:
This is our largest Uncharted game to date, and the team has been working incredibly hard to meet the challenge of closing out the game’s development in a timely manner. However, as we approached our final deadlines and started wrapping up the game’s levels, we realized that several key sequences needed extra resources to bring them to the finish line. After carefully considering all of our options, we decided to extend our schedule, making sure that we get a few more polish passes before submitting our gold master (the final disc for manufacturing).
They've gotta take their time to get the final chapter in Nathan Drake's story right, and that's alright with me.