The agreement, whose total value was not disclosed, was concluded in Moscow over the weekend during the visit of China's new President Xi Jinping. Russia was the first foreign destination for Xi after becoming China's top leader earlier this month.
The four submarines China plans to buy are the Lada-class vessels, China Daily and China Central Television reported. The four will be jointly designed and built by both countries, with two of them to be built in Russia and the other two in China.
China Daily quoted Chinese observers that any concern among region players about the arms-sales deal is "unnecessary" as it is not directed at any third party. It is the first time in about 10 years that China has gone for such a large military technological equipment deal from Russia, the report said.
"The Su-35 fighters can effectively reduce pressure on China's air defense before Chinese-made stealth fighters come online," the CCT reported.
Li Hong, secretary-general of the China Arms Control and Disarmament Association, said the recent purchases and joint building plan serve as an indicator of the evolution of the overall Sino-Russian strategic partnership, China Daily reported.
"It is the natural, well-deserved fruit of bilateral defense cooperation, and both sides have made it clear that the bilateral strategic partnership is not targeting anyone," Li said.
During his visit to the Russian Defense Ministry on Saturday, Xi, who is also chairman of China's powerful Central Military Commission, said he hoped their countries would strengthen cooperation between their military forces, the official Xinhua News Agency reported. The Chinese leader said he visited the Russian Defense Ministry on the suggestion of his Russian counterpart Vladimir Putin.
The world is still unequal, unbalanced and tumultuous, with challenges of both "traditional and nontraditional" threats, as well as the further spreading of turmoil in some areas, Xi said.
Russia's RIA Novosti reported Xi was the first foreign leader ever to be allowed inside the "heart" of the Russian military establishment.
"My visit to the Russian Defense Ministry is intended to confirm that military, political and strategic relations between the two countries will strengthen as will cooperation between the Armed Forces of China and Russia," Xi was quoted as telling Russian Defense Minister Sergei Shoigu.
Russia and China have held a number of joint military exercises since 2005. Both are members of the Shanghai Cooperation Organization, which also includes the Central Asian countries of Kazakhstan, Kyrgyzstan, Tajikistan and Uzbekistan.
China's growing military will have a budget of $114.3 billion this year, up 10.7 percent from last year as officially announced. However, outside experts have said the actual spending would be much higher than the official figure.