> It's all about height.
Except that the FCC doesn't take height into the ERP calculation. ERP = Power + Antenna Gain as the FCC sees it. Their regulations list that a station must lower its ERP when radiating at a higher height above average terrain.
I'd also LOVE to see a comercial radio/TV outlet that uses 4,000 watts to get 100,000 watts ERP. That's quite a fantastic gain figure, especially considering most commercial antennas are circularly polarized for greater signal penetration. Look up the antenna models from FCC queries, and you'll find the best omnidirectional ones offer a 6dbi gain -- enough to turn 4,000 watts into about 16,000 watts.
As a third point, DTV stations routinely run at 500,000 watts ERP in many urban areas of the US. Suddenly, the 400,000 watt figure is more plausible.