Don't know; that's why I asked.
The P4 contains about 55 million transistors. (
Reference here.)
The frequency rating is just what you'd think it is: the maximum frequency that the chip as a whole can operate at.
It's not so much a matter of safety as it is of the speed limits of the chip's circuitry. At gigahertz frequencies, it's operating in the radio-frequency (RF) band, where electronic devices reach their limit as to how fast they can switch on and off. Unlike RF devices, though, microprocessors operate entirely on square waves (at least ideally), so the gates and other devices in the chip are mostly transitioning between two states, on and off. (Even this isn't strictly true, as some circuits, like Schmitt triggers, have a range of voltages which they operate within.) But for a first-order explanation, you can look at the chip as operating entirely in the digital realm, using only square waves.
When the speed limit of a circuit is exceeded, it starts behaving unreliably, which obviously is unacceptable in a digital computer. So chips like the P4 are specced to operate below their absolute upper speed limit so that there won't be any "lost bits" or other glitches.