I've found these tweets from someone who has seen 4.13 already as they write reviews and stuff here's a link to their twitter:
link
The merlin tweets start from around 15 hours Vor now, there are also other tweets that have been made stating that gwen is in the ep all the way through, and the insgesamt episode is epic, soppy, sad and has a big ohhh moment.
I've added an image that has the best arwen related news and i think most of Du will be happy - i know i was :o)
