Legend of Korra's co-creator Bryan Konietzko has posted the following image on his Tumblr:
Korra fans are freaking out all over the world right now, but don't worry, the show is not being cancelled! There is a fourth season already on order by Nickelodeon. It's just moving to a new format, most likely just available on Nick.com or their digital smart phone apps.
The third season (or Book Three) of the Avatar: The Last Airbender spin-off has produced below average viewership numbers for the network. As a result, kids TV news site NICKandMORE reports that one episode from this Friday and four from August 1 and 8 have been pulled from Nickelodeon's programming, though Episode 8 “The Terror Within” is still scheduled to air this Friday at 8pm ET/PT.
We will learn more after the Korra Panel at SDCC. Once they give us some updates I'll make sure to run right back here and let my fellow Korra fans know.