Update: Video from the set added. Confirms much. Video credit to Marie, follow her on Twitter - @mcalfo!
There's a lot of rumors/unconfirmed spoilers going around about OUAT's season 4 finale. All reports are from the fans, being present on the set. Spoilers below:
Multiple people have reported seeing Emma holding the dagger. Regina, Robin and Hook are also close by. She apprently sacrificed herself to save someone/or everyone. You can hear her say: "we didn’t work this hard to lose" and "our happy endings”. Emma and Hook kiss. At the end, she disappears. Snow's crying.
There was still 4-5 hours of filming after that scene.
Video (confirms much from the above):