LEGO® Minifigures™ Series 4 were introduced in April 2011. Collection includes: The Monster, Surfer Girl, Sailor, Kimono Girl, Punk Rocker, Viking, Ice Skater, Hockey Player, Street Skater, Hazmat Guy, Musketeer, Werewolf, Artist, Crazy Scientist, Soccer Player, and Lawn Gnome.
The idea behind collectable Minifigures™ was that each minifigure come in a sealed bag, so you never knew which minifigure you might get. You can reed more details on Series 1 here. You can read about series 2 here, and series 3 here.
Detailed information can be found on LEGO® website here.
Although released in 2010, not available any more You can still try to find them on eBay.
Some more information on this series can be found on Brickipedia.
Below you can find photos of all Minifigures in this collection:
