I also think it is 4m Deathly Hallows coz i've seen this in a trailer.I don't know whether its after Ron leaves coz they seem so happy abt something, in the tralier,they were claping hands and hugging....
this scene in the deathly hallows when ron left harry and hermione but i thinck they don't really dance its just behind the backstage of the deathly hallows