I'm not sure what you're asking, but I'll take a stab at it.
You can do confirmed standby within four hours of the flight you want to take. The time of your original flight makes no difference. All that matters is: (1) what time it is now, and (2) what time the flight you want to take is scheduled to depart.
In contrast, there is no four-hour rule for regular (i.e., not confirmed) standby. For regular standby, you just put your name on the list and wait at the gate for a boarding pass.
Well your flight does matter because I presume you can't make the call if your flight should have left already... you'd be on the flight or maybe trying to do a flat-tire on purpose.