<< because the 4% mark has appeared to be the lowest we can go without seeing inflation perk its evil little head. >>
Might want to stop reading some of those older textbooks. NAIRU (Non-Accelerating Inflationary Rate of Unemployment) has been thoroughly discredited. The old "maximum threshold" for unemployment used to be 6%, and as unemployment steadily dropped through the 90s without even a hint of inflationary pressure (indeed, price pressures were often deflationary), the professors have been steadily revising the threshold down. Now, the supposed number is 4%. Why should we believe them at 4% when they were wrong at 6%? That's a 50% discrepancy....
NAIRU sounds nice as a theory and seems to make sense, but has a fundamental flaw - it assumes human capital is a commodity and a 'parity product', that is, all unemployed persons are equal, and disregarding skill sets and geography.