Cao Junjun, one of two diplomats in Papua New Guinea, said the three men and a woman "were killed by masked criminal at around 9 p.m. local time on Monday," the official Chinese news agency Xinhua reported.
The (Hong Kong) Standard reported the four were employees of the bakery.
The only Chinese survivor said some of cash in the bakery had gone missing, the diplomat said.
Papua New Guinea Prime Minister Peter O'Neill Wednesday said the attack was "brutal and cowardly," and an investigation was under way.
Wang Yanbo, the second Chinese diplomat, said the attack "is a pure criminal case."