Who is Mr. X? (S04E09)

S04E01 The Longest Good-Bye S04E02 Here's Looking at You, Kid S04E03 Death of a Fruitman S04E04 Change is Gonna Do Me Good S04E05 The Heavyweights S04E06 Carol's Wedding S04E07 Shrinks Across the Sea S04E08 What's It All About, Albert? S04E09 Who is Mr. X? S04E10 Seemed Like a Good Idea at the Time S04E11 Over the River and Through the Woods S04E12 Fathers and Sons and Mothers S04E13 The Article S04E14 A Matter of Vice-Principal S04E15 Bob Has to Have His Tonsils Out, So He Spends Christmas Eve in the Hospital S04E16 No Sale S04E17 Carol at 6:01 S04E18 Warden Gordon Borden S04E19 My Boy Guillermo S04E20 Duke of Dunk S04E21 Guaranteed Not to Shrink S04E22 Birth of a Salesman S04E23 The Boy Next Door S04E24 Peeper Two