Yes. In the season finale, the box under the Tree of Harmony opened and the ponies transformed into their Rainbow Power equivalents. They looked exactly like the toyline this design spawned from.
Yes, the official synopsis of the Season 4 Finale hints at the "Rainbow Kingdom".
It is not known what these flashes of rainbow in the pony's eyes is supposed to represent, but this will likely be revealed in the Season Finale of Season 4 of My Little Pony: Friendship is Magic.
The Crystal Empire is not yet considered to be the Rainbow Kingdom, but is suspected that is will be designated this title in the Season 4 Finale.
This information has not been revealed yet. In the Season 4 Finale, this information will be available.
yes
It has been suggested by the synopsis of the Season 4 finale and by press releases from Hasbro that the Crystal Empire is in fact the "Rainbow Kingdom". What this means is unknown.
Probably not. It has been said by the production team of Equestria Girls that Equestria Girls was meant to be a standalone, and not be connected to the TV show My Little Pony: Friendship is Magic.
No. There will be a season 5.
Yes, the Season 4 finale of your Little Pony Friendship is Magic will be a two partner.
Season 1, Episode 1: Friendship is Magic
There are 13 episodes in season 3.
Season 5 of My Little Pony: Friendship is Magic is the most recent season to be announced. It is set to be released later in 2015. As of February 2015 there is no information about any additional seasons to come.