Sam Westing was disguised as Sandy McSouthers, until he "died" on the day that the final part of Sam Westing's will was revealed.  It was wintertime, not actually the Fourth of July, but the Westing house was blown up by fireworks so that it looked like the Fourth.  On that day Sandy faked his death, and only Turtle knew that he survived under the pseudonym Julian R. Eastman. 

The heir who wins the windfall will be the one who finds the fourth.  It was so simple once you knew what you were looking for. Sam Westing, Barney Northrup, Sandy McSouthers (west, notth, south).  Now she was on her way to meet the fourth identity of Windy Windkloppel.  She could probably have figured out the address, too, instead of looking it up in the Westingtown phone book -- there it was, number four Sunrise Lane...Julian R. Easman rose.  He looked stren.  ndvery prope.  He wore a gray business suit with a vest, a striped tie.  His showes were shined.  H limped as he walked toward her, not the crooked limp of Doctor Sikes, just a small limp a painful limp.  Again Turtle was gripped by panic.  He seemed so different, so important.  She should have kicked him (the Barney Northrup him).  He was coming closer.  His watery-blue eyes stared at her over his rimless half-glasses.  Hard eyes.  His teeth were white, not quite even (no one would ever guess they were false).  He was smiling. He wasnt' angry with her, he was smiling. 
       "Hi, Sandy," Turtle said.  "I won!" (174-5)

Source: Raskin Ellen.  The Westing Game.  New York: Avon, 1978.