North Korean news agency KCNA said the launches were an exercise designed to prepare the country for a strike.
Supreme Leader Kim Jong-Un had overseen the launch, the agency said, and had lauded what he had witnessed.
KCNA said the exercise was carried out by an artillery unit “tasked to strike the bases of the US imperialist aggressor forces in Japan in contingency”.
The four missiles flew an average of 1,000 km (620 miles) and reached an altitude of 260 km (160 miles), Japan’s Defence Minister Tomomi Inada said.
Some landed as close as 300km (190 miles) from Japan’s northwest coast.
Under UN resolutions, Pyongyang is banned from using ballistic missile technology.
Washington and Tokyo have called for the United Nations Security Council to meet over the launches, which is expected to take place on Wednesday.
President Trump has previously said he will not allow the North to obtain a nuclear weapon that could threaten the US.
So far, he has yet to state how he intends to stop it.
His sentiments were echoed by the US ambassador to the UN, Nikki Haley, who said on Twitter that the world “won’t allow” North Korea to continue on its “destructive path”.
The missile launches came amid annual military drills involving US and South Korean armed forces that the North regards as a rehearsal to an invasion.
Pyongyang warned that the military exercises, which it called “the most undisguised nuclear war manoeuvres,” are driving the Korean Peninsula and northeast Asia toward “nuclear disaster.”
The White House said earlier the US would be taking steps to boost defences against ballistic missiles, including the deployment of the Terminal High Altitude Area Defence (THAAD) defensive missile system in South Korea.
The system was originally scheduled to be installed in autumn, but a US official said it could be brought much further forward.
Japan has said it also plans to reinforce its missile defences and may buy either THAAD or a ground-based version of the Aegis system deployed on US warships.
The missiles were launched from the Tongchang-ri region, near North Korea’s border with China, South Korean military spokesman Roh Jae-Cheon said.
UN Secretary General Antonio Guterres condemned the missile launches and urged Pyongyang to refrain from further “provocations”.
He said in a statement: “Such actions violate Security Council resolutions and seriously undermine regional peace and stability.”