1) After a civil war that had lasted for nearly 10 year the khmer rouge took power in 1975 and stayed in power until 1979. In the end they fell out of power because their troops breached the boarder to vietnam in order to hunt down people opposting them. After the war the country was under UN mandate in 1993 there were democratic election. Today when one speaks of Cambodia and communism one usally means the reign of the Khmer rouge exept something esle is stated explisitly.
2) sucess? Killing people faster than Hitler ever managed. Destroying the whole elite of a country everyone with glasses counted as intellectual along with the old elite, putting it back to stone age. thats why it is also called "stone age communism" a totaly unique form of communism that wanted to establish a purely agrarian society. Destroying families, ....
3) Hun Sen as prime minister. Norodom Sihamoni, as king
4) the country is not communistic, although from my friend in vietnam i know that the khmer rouge and affilated groups still life as terror groups in the mountains. - Much to the annoynac of everyone in the region. Economically the capital and the areas around it do well, but there is a lot of powerty on the countryside, especially in the mountain areas.