Governor Romney has said that 500,000 is the monthly job creation he expects as normal during a recovery . It actually last happened in May 2010 but is otherwise pretty rare. Reader Rick Stryker argues that the 500K should be expressed in percentage terms, accounting for the size of employment. Nonetheless, breaching the equivalent in percentage terms never happened during the eight years of the G.W. Bush administrations.
Figure 1: Month on month growth rate (log differences, not annualized) in nonfarm payroll employment, s.a. (blue), and 500,000 monthly change, expressed in percentage of 2012M04 NFP. Source: BLS April release, via FRED.
In fact, over the last 21 or so years, one has to go back to the Clinton administrations to find such numbers.
Oh, and by the way, unemployment has been at or below 4% only 11 times in the past 40 years (essentially at the end of the Clinton Administration).