Which area is classified as Region IV in the Central Valley's heat summation zones?

Region IV in the Central Valley's heat summation zones is specifically characterized by a climate that is conducive to growing a wide variety of grape types, especially those that thrive in warmer conditions. The Central Valley, being a significant agricultural area in California, benefits from higher temperatures and lower rainfall, which helps in the ripening of grapes and the production of wines.

This classification reflects the area's ability to produce grapes that often have full phenolic ripeness and elevated sugar levels, ideal for making bold wines. Other areas listed in the options, like Sonoma and Napa Counties, while they do have their own unique climates and might have different classifications, are not classified as Region IV within the context of the Central Valley's specific heat summation zones. The Coastal Range, being a more temperate and cooler region, also differs significantly from the warm climate characteristic of Region IV in the Central Valley.

Thus, the classification of the Central Valley as Region IV aligns with its geographical and climatic attributes central to viticulture, particularly in the context of heat summation which plays a critical role in determining the types of grapes that can flourish there.