The North, which often fires short-range missiles as part of military drills and usually times the launches for periods of diplomatic friction, was hit with UN sanctions following its May 25 nuclear test.
The salvo began with two surface-to-ship missiles fired off North Korea's east coast between 5:20 p.m and 6 p.m. (0820-0900 GMT) that flew about 100 km (60 miles) and splashed into the sea, a South Korean defense official said.
A third short-range missile was fired around two hours later, the defense ministry said, and South Korea's Yonhap news agency, citing officials in Seoul, later said a fourth had been fired.
North Korea last month warned shipping to keep away from a maritime zone extending 110 km off its east coast between June 25 and July 10, saying it was conducting a military drill.
This activity is not unexpected, said U.S. Pentagon spokesman Bryan Whitman.
North Korea continues to develop and pursue missile technologies and the United States continues to remain concerned about not only their missile activities but their proliferation activities and their nuclear program.
A South Korean daily said that the secretive North may also test fire mid-range missiles, viewed by the South, the United States and others as a more serious act, in a matter of days.
Japan, a party to currently suspended six-nation talks aimed at coaxing the isolated North to give up its nuclear program in return for aid and greater diplomatic recognition, was quick to condemn Pyongyang's latest action.
We have often warned that such a provocative act is not beneficial for North Korea's national interest, Kyodo News Agency quoted Japanese Prime Minister Taro Aso as telling reporters.
The short-range missile launches came after regional markets had closed for the day, but East Asian investors have grown used to North Korea's saber-rattling and tend not to be fazed.
Analysts say they would likely panic only if there was military conflict on a peninsula, where 2 million troops face each other across one of the world's most heavily armed borders.
Washington said this week it had tightened its crackdown on firms linked to the North's lucrative proliferation of missiles, a major source of cash for the destitute state, and has sent the U.S. point man for sanctions to Asia for discussions.
Enforcement of the sanctions, aimed at halting its trade in arms, would depend heavily on China, the North's biggest benefactor and trade partner, analysts said.
China said on Thursday it was sending its envoy to the six-party talks to South Korea, Japan, Russia and the United States. North Korea, the sixth party, was not on the itinerary.
China has consistently advocated dialogue and consultation, and achieving denuclearization of the Korean peninsula through the six-party talks process, Foreign Ministry spokesman Qin Gang told a news briefing.
Earlier the JoongAng Ilbo daily quoted an intelligence source as saying the North was likely to fire medium or short range missiles from its east coast in early July that could include Scuds with a range of about 340 km (210 miles) or Rodong missiles with a range of up to 1,000 km (620 miles).
North Korea fired a barrage of short-range missiles following its May nuclear test, which experts said put the state closer to having a working nuclear bomb.
It launched a rocket in April in what was widely seen as a disguised long-range missile test that violated U.N. resolutions banning it from ballistic missile launches.
Philip Goldberg, the U.S. envoy who coordinates sanctions against the North, has been in China to enlist Beijing's help in getting tough with North Korea and said he had had good discussions with his Chinese counterparts.
We want all the various aspects of the resolution to work, he told reporters after a day of meetings, adding that this included financial sanctions.
He will be in Malaysia on Sunday before heading back to Washington on Monday.
It was not immediately clear why he was visiting Malaysia, although earlier this week Japanese media reported police in Japan had arrested three people on suspicion of attempting to export equipment that could be used in weapons production to Myanmar via Malaysia and suspected a link to North Korea.
South Korean Foreign Minister Yu Myung-hwan said he was seeking a meeting of the foreign ministers of the six countries, including the North, on the sidelines of a regional security forum on July 23 in Thailand.
Officials said the North's military grandstanding is likely related to moves by its leadership to begin readying leader Kim Jong-il's youngest son as a future heir by consolidating the 67-year-old leader's power base.
(Additional reporting by Chris Buckley and Emma Graham-Harrison in Beijing, Kim Yeon-hee and Jon Herskovitz in Seoul, Yoko Kubota in Tokyo and Andrew Gray in Washington; Editing by Jeremy Laurence and Alex Richardson)