Question: are apple watch series 4 bands compatible with series 3?
Answer: You can use a band that's designed for Apple Watch (1st generation) and Apple Watch Series 1, 2, and 3 with Apple Watch Series 4 and later as long as the sizes are compatible. Bands for 38mm and 40mm cases are compatible with each other, and bands for 42mm and 44mm cases are compatible with each other.