A newspaper story showed that the Soetoro’s married in 1965. Obama was four and could have been adopted and become a citizen without the need for naturalization. The story about him being 5 or 6 came out later.
But they did not move back to Indonesia until Obama was 5 or 6 and I don't see how an Indonesian adoption could have been accomplished outside of the country especially when the law said that the Pengadilan Negeri, which I assume is a government entity, had to certify the adoption at the residence of the adopted father. In 1965 the residence was Honolulu.