If you're sitting on top of it, even a 4.0 can get you attention.
Having been in SoCal for both the Sylmar quake and the Whittier quake, plus a few others, I do not take serious earthquakes lightly.
However, a 4.0 will rattle windows, and yes, you will feel it. But almost no serious damage will result. Earthquakes are normal shifting action of the earth, as you well know, and 4.0 is not newsworthy, unless:
a) there is an unusually high number of quakes occurring at close intervals in time (which may be the case here) or
b) Some one is killed or serious damage occurs.
c) It occurs in an area where earthquakes rarely happen.
I think you can compare earthquakes of low magnitude (below 3.0 ?) to strong whirlwinds, that cause no damage, vs. a strong earthquake (6.0 and up) that results in severe damage and possible death, which would be more like a full blown tornado. Whirlwinds do not make the news, tornados do.
In between 3.0 and 6.0 may be a judgment call, I admit. But it seems to me that in a strongly active earthquake zone (Ca., Alaska), a 4.0 does not merit too much attention.