Santa Rosa can best be thought of as a city with four large neighborhoods (or quadrants) and a downtown in the middle. This is because Highway 101 runs north - south through the middle of Santa Rosa, dividing the city into an eastern and western half, while Highway 12 / 4th Street runs east – west and cuts Santa Rosa further into a northern and southern half.
There are three parts to Downtown Santa Rosa: 4th Street (which is what most people mean when they say "downtown"), Historic Railroad Square and Santa Rosa Plaza shopping mall.