Sherlock ended its fourth season — and possibly its entire series — with the intense “The Final Problem,” which put Holmes and Watson through the emotional wringer, explored the great detective’s mysterious childhood, and ultimately set the stage for a more traditional version of the iconic mystery tales moving forward (should Benedict Cumberbatch and Martin Freeman ever agree to return).
This interview covers some of the scenes in the finale including Molly's, the plane flashback, etc. They don't really reveal much information and leave it pretty open ended as to whether they will come back with another season:
There are no cliffhanging items left over, not that I can think of? No. There isn’t something we have to come back to address. Which is quite good for us. Because if we do come back, we can just start with a knock on the door and a new client, and they can go and investigate.