King Philip IV of Spain reigned from 1621 to 1665 and was a member of the House of Habsburg. His rule is often associated with the decline of Spanish power, as the country faced military defeats and economic challenges during the Thirty Years' War. Philip IV was known for his patronage of the arts, supporting famous artists like Diego Velázquez. His reign also saw the rise of Bourbon influence in Spain, which would later lead to significant political changes in the country.