Three maritime surveillance vessels entered the 12-nautical-mile zone around one of the islands in the East China Sea shortly after 6:30 am (2130 GMT Wednesday), the Japanese coastguard said in a statement.
Another surveillance ship joined them one hour later.
The four Chinese vessels were off Minamikojima, one of the islands in a chain known as the Senkakus in Japan and the Diaoyus in China.
Separately, two fisheries patrol ships were spotted in the so-called contiguous waters, which extend a further 12 nautical miles, of another island in the chain, coastguards said.
The two vessel types are run by different Chinese government agencies but are not military.
Japanese Vice Foreign Minister Chikao Kawai "strongly protested to the Chinese ambassador by telephone about the Chinese ships' intrusion into Japan's territorial waters," the foreign ministry in Tokyo said in a statement.