The four latest suspects are Jacob Hess, 18, Joshua Hess, 19, Sarah Couch, 18 and Brandy Sneider-Koontz, 19.
All four juveniles have been charged in several arson-related counts related to the Washington County and Frederick County fires.
The investigation, which began one month ago, produced a total of eight arrests.
The series of blazes charred vehicles, garbage containers, a barn and a garage and caused over $130,000 in damages altogether.
A Washington County Grand Jury will hear from the first four suspects – Tyler Murray, Sarah Mathews, Morgan Nield and Olivia Hannah – on April 30th.