4'33" isn't "silence", it's "sounds of an orchestra playing nothing". It's supposed to be the sounds of pages turning, people shuffling for comfort, so on and so forth. A countdown clock would not be an appropriate replica.
Watching a blank video has a similar effect: You start to hear the ambient sounds... traffic outside the window, the ticking clock in the corner, the 60Hz hum of your electronics, the creaking as you rock in your chair, air whooshing through the conditioner vent...
4'33 is okay as an experiment, sort of like an unexpected meditation/awareness of the present class to an unsuspecting audience (for those hearing it for the first time). Not much beyond that, though.
