English: Built during Period IIIa-IIIb around 450-500 CE atop earlier constructions, System IV features talud-tablero (slope-panel) constructions similar to those found in Teotiuhacan, as well as numerous overlapping facades in the style known to archaeologists as "double-scapular" (doble escapulario); the shape of these linear moldings is thought to be evocative of symbols of the sky. System IV's courtyard patio may have served as an amphitheater for observers of rites conducted on the main pyramid, while a niche, perhaps used for offerings, is located at the central base of the outer eastern stairway of Mound N. Two tunnels extend under the surface of the complex. Some of System IV's earlier construction elements that remain intact include a six meter-tall sloping wall dating to Period I (500-100 BCE) and a temple in the northwest corner that dates to Period II (circa 200 CE). System IV is a temple-patio-altar complex (like System M), with a central altar and stairs to the east and west at the edges of a private patio area cut off from outside view. Adobe walls, probably topped in the past with flat wood/earthen roofs, rest atop stone foundations.