The United States Geological Survey says the quake hit at 2:23 a.m. Saturday about 24 miles north of Santa Rosa, or about 76 miles north of San Francisco.
The USGS had initially reported the quake as having a 4.3 magnitude, but issued a revision after it was reviewed by seismologists.
Some residents reported feeling light shaking, but a sheriff's dispatcher says there were no calls of damage or injuries.
The area where the earthquake hit is near the Geysers geothermal development, a complex of 22 geothermal power plants in the Mayacamas Mountains. The area experiences frequent seismic activity.