In an era when the mobile phone has become the paintbrush of choice of many contemporary artists, it may seem inappropriate to talk about "extreme" art. But as the artist of the 21st century dials up the world's best waxwork-maker and asks him to make a perfect replica of Hitler as a nine year-old boy, or flies in to his studio in Mexico to sign another 40 new paintings, which he hasn't made himself, or as the artist hires a team of construction workers to fill a gallery with mud, they are still going to extremes.

Here we profile four artists that are working on the edge.