Gregory Dow, 61, admitted during a federal court hearing in the state of Pennsylvania to having committed the crimes between 2013 and 2017.
Under the terms of a plea agreement with prosecutors, Dow would receive a prison sentence of 15 years and eight months.
The charges against Dow stated that when he began abusing the girls, two of them were 11 years of age, one was 12, and one was 13.
Dow "purported to be a Christian missionary who would care for these orphans," US prosecutors told the court.
Dow had pleaded guilty in 1996 to sexual assault in the US state of Iowa.
