Dr. Mason (aka Number 10) was one of the forty applicants who applied for the three open fellowship positions in Season 4. He was portrayed by Jonathan Sadowski. He lasted until about the middle of the first episode to feature the applicants, The Right Stuff.
Dr. Mason was first assigned to do an environmental scan of Greta Cooper's apartment along with Dr. Ashka and Henry Dobson. He attempted to jimmy open the window to Greta's apartment, only to find out that Henry had obtained entry by saying he was Greta's uncle.
However, this success proved to be his undoing. When Lisa Cuddy found out about the patient, she demanded the applicants tell her who the patient was. Most of the applicants didn't know, but Dr. Mason finally broke down and told her who she was. He had found out her name and occupation by looking through her mail looking for diagnostic clues.
When Dr. House returned, he figured out who had ratted him and the patient out to Cuddy and fired Dr. Mason. House had already figured out it had to be Mason, Ashka or Dobson as they were the only persons who had any access to the patient's personal information. He also surmised Dobson couldn't be the culprit because he was busy napping, and Ashka wouldn't have done it because of her immigration status. He also noticed that Amber Volakis was discreetly pointing at Dr. Mason to try to eliminate him from the competition.