It Ain't nessicarilly so....
"I know I'm compromising and that REAL mastering monitors are larger and most expensive, but pretend your in my position!"
Most mastering facilities utilize more than one set of monitors. 4-6k is pretty reasonable for a set of nearfields, and a good start. Take a peek at some mastering websites, most will list montors, or atleast brands, on their site. Something differant than what you use for mixing is essential, you dont need to have a monitor that survives a snare at 100 dB through these. A differant room is even more important.