The four vessels sailed into the 12-nautical-mile band of territorial waters around 7:30 am (2230 GMT Tuesday), the coastguard said.
It is the latest in a series of incursions by Chinese government ships in recent months around the Senkaku islands. Beijing also claims the islands, which it calls the Diaoyus.
Japan on Tuesday unveiled its biggest warship since World War II, a $1.2 billion helicopter carrier aimed at playing a major role in disaster and rescue missions, as well as defending sea lanes and Japanese territory.
The dispute over the island chain in the East China Sea has been cited as a potential flashpoint that could even lead to armed conflict between the Asian giants.