I'm not 100% sure, but I heard that the last time the fourth chime was used on the air was in 1986 on both NBC radio and TV, when it was announced that NBC parent RCA was being sold to General Electric.
I don't know if it's true, but I have also heard an "urban legend" that the fourth chime was sounded on a closed-circuit basis to NBC television affiliates on the morning of September 11th, 2001 to urge them to switch to the East Coast feed of "The Today Show", a couple of minutes after the first plane struck the World Trade Center, as the East Coast feed of that show was coming out of a commercial.
Actually, one recent occasion when NBC could have used a fourth chime on it's TV network was at the beginning of the Special Report this past April 15th covering the bombing at the Boston Marathon. Today, NBC television Special Reports begin with the three chimes, perhaps an opening with four chimes could be prepared and used for only "transcendent" stories like this one or the capture of Bin Laden. For less-important interruptions (read: a Presidential press conference or the announcement of the birth of Prince George), the usual "Special Report" opening with three chimes would be used.