Many had tipped the 22-year-old to reach the semi-final at best in his campaign at the Games.
However, he defied expectation to not only advance through to the final, but finish just one place outside of the medals.
"I can't believe I've come fourth in an Olympic Games. I clashed arms with a South African, so maybe I could also have got a PB. I treated the semi as the final," he told BBC Sport.
"I'm really pleased with this and the crowd was absolutely amazing. The last four years have been amazing. I just can't believe it."
Clarke won gold at the European Junior Championships in 2009.