War won't break out between South Korea, USA, Japan and North Korea. Neither will Situation deescalate in the foreseeable future. All parties in this conflict profit from the current tension.
USA: The conflict helps them justify their presence in the area (military bases in Japan, South Korea and on Islands in the south pacific). It also allows them to let more funds dissapear in to the black hole which is their military budget.
Japan: A united Korea's economic and military potential threatens Japanese regional and international interests. They must keep the conflict alive by tolerating US-presence. It would be far more beneficial for a united Korea to be allies with China, rather than with Japan and the US.
China: They are NK's main supplier of goods and resources. They want to prevent a war. If the Situation were to escalate they would have to expect 20+ Million refugees crossing the NK-China border.
South Korea: In case of war, Seoul would be reduced to ashes. SK politicians use the conflict to their advantage, securing (re)election by adopting a firm stance towards North Korea. The USA is an important trade Partner they don't want to annoy (more than 20% of SK export).
North Korea: Kim needs the conflict to rally his people against a common enemy, so he can divert attention away from the shortage of resources and poverty that plague his country. Also, he must prove to the North Korean elite that he is a strong leader. He needs the support of his military. Dictatorships have often fallen due to military coups.
Please excuse any mistakes. English is not my first language.