What was Westing's disguise?

Let's start with the easiest disguise. Sam Westing's first "disguise" is himself. Readers and characters don't get to interact with this particular version of Westing, since everybody believes that he was murdered. Savvy readers might have been able to pick up on Westing's other disguises. During chapter 26, a really big hint about Westing and his disguises is dropped.

"The heir who wins the windfall will be the one who finds the… FOURTH"

I'll be honest, I didn't understand that clue, but Turtle Wexler did. The fourth refers to Westing's four identies, but the fourth also refers to the four cardinal directions: north, south, east, and west. Sam Westing, Sandy McSouthers, Julian R. Eastman, and Barney Northrup.
Readers get very little time with Julian Eastman. Turtle meets him in the end, and he is the chairman of the board of Westing Paper Products Corporation. Northrup has a mustache and is the realtor for Sunset Towers. Sandy McSouthers is the most visible of Westing's character creations. He is the poorly-educated doorman at Sunset Towers, and he is always working to lead other characters to make certain connections and doing certain things.