<< P4's start to throttle down about 40 degrees celcius hotter than they run under load. >>
Is this just a guess or do you actually have any documentations as to this?
Either way, It's pretty safe to say that clock throttling will NOT be an issue unless your running without a HSF, or with extremely minimal cooling.
As a rough guess I'd imagine it would activate at a temperature fairly substantially lower then it's maxiumum rated tolerance, but also quite highly above any viable temperatures likely to be seen with retail cooling.
My guess would be approximately 78C sustained or so... though it likely varies slightly depending upon it's rated clockspeed.