Viscosity has nothing to do with API service category. They are completely independent. The SAE "weights" are based on the oil's viscosity at 0C and 100C. Because higher viscosities are usually specificed for diesel engines compared to spark ignition engines, 15W-40 is the most common multi-viscosity CI-4 grade; 10W-30 is also available, but not commonly stocked in warm climates, and synthetic base CI-4s are available in 5W-40 if you have to do subzero cold starts.

Prior to switching to CI-4, I was using 20W-50 year round.

A 15W-40 is certainly a "year round" oil on the Gulf coast, most of the Southwest and the Pacific Coast.

Duke