Researcher Fred Kraus from the Bishop Museum in Honolulu said the frogs belong to the genus Paedophryne, all of whose species are extremely small.
"Miniaturization occurs in many frog genera around the world," he said, "but New Guinea seems particularly well represented, with species in seven genera exhibiting the phenomenon."
Kraus discovered the first species of Paedophryne in 2002 from nearby areas in New Guinea.
The four known species -- including the two new smallest, Paedophryne dekot and Paedophryne verrucosa -- all inhabit small ranges in the mountains of southeastern New Guinea or adjacent, offshore islands, he said.
"Although most frog genera have only a few diminutive representatives mixed among larger relatives, Paedophryne is unique in that all species are minute," Kraus said.
The finding was published in the open access journal ZooKeys.