The fourth most watched T.V. episode of all time:

Fugitive final episode
The final episode of The Fugitive in August 1967 won a 45.9% share of the audience.
Source: USA Today, 5/25/1999

 
   
  The fifth most watched T.V. episode of all time:

Cheers final episode
The conclusion of "Cheers" in May 1993 achieved a 45.5% share of the audience.
Source: Christian Science Monitor, 5/8/1998