EDIT 1: About the builder bear thing....the toys were displayed under a sign bearing the words 'Rainbow Fun'....so I guess we all know what that means....
EDIT 2: I watched the season finale....I. TOLD. YOU. SO. I'm always right. Most of the time. Some of the time. Every 10 years.
'Rainbow Power' is probably now a phrase that most MLP fans will recognise. I have quite a few thoughts on the matter, so I thought I'd better put it down. Time for some thorough analysis and philosophy, folks!
There's this shop near to where my Aunt lives that sells brochures and magazines and newspapers and things like that and she knows that I'm a fan of MLP, so she decided to get this brochure booklet thing for free and give it to me. It has stickers and quizzes and puzzles which are all normal for promotional booklets like that, but I noticed all the pictures were of the Mane 6 in their Rainbow Power form. It had the Rainbow Power logo on it and promoted the Rainbow Power line of toys, of which I have a Fluttershy and Rainbow Dash. They are pretty much standard pony toys except that they have tinsel in their manes, so don't feel that your missing out on much.
There are also quite a lot of adds on TV here in the UK, but there are probably loads on the Hub or something in America. Most of them are promoting the toys for Rainbow Power, and every single toy shop I visit it's all Rainbow Powe toys and not standard pony toys.. It's almost as if it has replaced the normal line of toys. They are doing MLP plushies at a Build-a-Bear store near where I live, but I have still to check wether those are Rainbow Power too. I'm probably going to be doing that discreetly, though.
And then there's Equestria Girls 2. The 'Rainbow Rocks' line of toys is pretty much the EG equivalent to the pony toys. And the new film that's coming out later this year was either made as part of the Rainbow Power concept or later developed as a way of advertisment, I guess. In the trailer we have at the moment, it shows the EG's in their ponified-ish forms with the Rainbow Power trademark, which is crazy coloured strands in their hair. They also have their cutie mark on their face, so that could represent the ponies having their cutie mark enhanced(?) in their Rainbow Power form.
What relation does Rainbow Power have with the show or Season 4 in particular? Have you noticed that on the logo of Rainbow Power and the colours used to highlight the ponies main and tails, matches the rainbow colours we see in the episodes the ponies get their key? And remember that box thing in Princess Twilight Sparkle part 2? It's just a theory, and theorys are pretty much the only things we can come up with right now, but when they open the box, maybe they get this super cool rainbow power up? The rainbow colours also appeared in the same episode as the box when the tentacle-things got dispelled and also when the trail thing appeared leading to the box. I mean, what would be the point of doing the rainbow thing when the Mane 6 get their key plus bringing out a whole new line of toys as well as a movie if its not going to connect some way with the show? I don't really know why they would need the power up, maybe it will be a two-parter or something and they find a villain and only manage to unlock the box once the villain has been reavealed. I don't know.
That's what I've noticeed so far, but I don't really agree with the way they sell this thing. I mean, 'Rainbow Power'? It probably would help if they called it something else, like 'Harmony Power' or something, at least. They could have just called it 'Sparkle Power' or 'Apple Power' or 'Party Power'. That's just my opinion though.
If I've skipped anything out just say in the comments and I would also like to know if you guys have noticed these things too or if you agree or disagree in any way.
Plus the art of Rainbow Power I've found is pretty amazing.