Outkast rapper Big Boi and Australian pop singer Sia are among the artists confirmed for the 2011 Outside Lands music festival in San Francisco. The lineup for the fourth annual event, which takes place from August 12-14 in Golden Gate Park, was revealed on Monday.

Sia is a music fest veteran, having performed on several dates of the Big Day Out Festival in her home country. Big Boi, who guested on Janelle Monae's Grammy-nominated hit 'Tightrope,' is still touring in support of his 2010 solo record, 'Sir Lucious Left Foot: The Son of Chico Dusty.'

Grammy-winning rock bands Muse and Arcade Fire co-headline the fest along with jam band Phish. Among the other artists on the heavily rock-and-R&B oriented Outside Lands lineup are the Black Keys, MGMT, OK Go, Erykah Badu and the Roots.

Tickets for the festival go on sale Thursday, April 14 at 3PM EST.

Watch the Sia, 'Clap Your Hands' Video

Watch the Big Boi, 'Shutterbug' Video