Thanks to Michu1945 for the heads up.
Take the following with a grain of salt.
First we have this tweet that appears to come from someone who watched the filming from the set.
Looks like Emma sacrificies herself for Regina. Jen runs away at the end, like Emma disappears? Last scene of s4? #OUAT #OnceUponATime
— Marie ㋡ (@mcalfo) April 2, 2015
Then we have this photo, allegedly from the scene.
(click to enlarge)
Thanks to Nordica for this extra info.
Megaspoiler from OUAS:
Info from fans that are on set:
" Emma has the dagger. Emma takes Regina’s place. Emma and Hook apparently kiss before pushing him away, then lightning hits her/the dagger and Emma vanishes. Everyone is confused and Snow is crying.
Before the above happens, Robin tries protecting Regina from whatever is attacking her, but is thrown back.
After Emma disappears, the dagger is left behind.
Whatever is attacking Regina, Emma takes her spot. No body switching or anything. "
Info from fans that are on set:
" Emma has the dagger. Emma takes Regina’s place. Emma and Hook apparently kiss before pushing him away, then lightning hits her/the dagger and Emma vanishes. Everyone is confused and Snow is crying.
Before the above happens, Robin tries protecting Regina from whatever is attacking her, but is thrown back.
After Emma disappears, the dagger is left behind.
Whatever is attacking Regina, Emma takes her spot. No body switching or anything. "
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