power for 2.4Ghz 'WiFi' is actually limited by the EIRP (equivalent isotropically radiated power) with lots of details covered in FCC Rules and Regs title 47 part 15.247 provides details on limitations of EIRP.
The most common WiFi limit is +36dBm - which equates to 4W. However, this is TOTAL emissions, and any gain of an antenna must be accounted for. A Bullet attached to a 6dB antenna leaves only +30dBm of transmission
power allowed - or approx 1W. Use a more common 8dB antenna and you are already over the limit (though even the direct coupling gives a 1-2db loss - so . . maybe OK)
So, advertising a '4W Power' is perhaps a bit misleading. Of course you could use a zero-gain antenna, but just having a big mouth (powerful transmission) does nothing for radio
signals if they do not also come with good ears (receiving gain)
is MUCH lower and very aggressive seeking out violators I hear - more so in some countries. Australia
of all places is a bit higher
in the USA.