This reportedly came from someone who works directly for DHX Media, the company which produces animation on the show. These are no direct sources, however.
If true, this means a few things: First of all, season four has been confirmed! So has alicorn princess Twilight Sparkle, something that has been speculated in the fan community for some time. Hasbro will also be harnessing more control over the show come season four, which may mean changes in the overall tone and story.
This vector of alicorn Twilight was seen in the background of a Twitter photo from Brand Licensing Europe. Even if the DHX worker image does wind up being doctored or untrue, the idea of an official alicorn Twilight is not unfounded.
Derpy Hooves/Ditzy Doo will be no more, which means there will be no future toys produced of her. However, I am not sure what this means for adult fan merchandise, such as the products sold at Hot Topic and WeLoveFine. Get those vinyl figures while you can!
Again, this is just a rumor and nothing has been officially confirmed.
Thanks to MLP G4 Merch for this news. You can read more about these rumors at the source.