> So Jupiter hasn't been the solar system vacuum for billions of years?
Yes, it has. But 2004MN4 is in a reasonably circular orbit, very similar to Earths. Jupiter has served to hoover up the asteroids and comets in highly elliptical or hyperbolic orbits that come somewhat near it. What large planets like Jupiter do is take care of very eccentric orbits; Jupiters effect on circular orbits distant from it is just minor chaotic orbit tweaking.
Not sure this link will work right, but what the hey:
http://neo.jpl.nasa.gov/cgi-bin/db_shm?sstr=2004+mn4&group=all&search=Search
> Those comets and asteroids have been awfully plentiful on our short life spans.
As they have in all human lifetimes. In fact, it certainly seems that impressive, naked-eye comets have been in short supply for a few generations. I've only seen one or two naked-eye, and I would've missed both had I not known to look for them. Whereas the 1910 approach of Halleys Comet lit up the sky, as did many previous comets. The sky seems to have been rather quiet of late.
And that then sky has a lot more scatter from electric lighting than existed before 1900. How many people living in NY can actually see stars anymore?