Traditionally, a 3 season sunroom is not insulated and has no HVAC. The 4 seasons sunroom is built with insulated glass (often including Low E for keeping down solar heat gain) and insulation in the walls and ceiling. While a 3 seasons porch is going to get you through most of the year including the moderate portions of the hotter and colder months – certain times of year may be less enjoyable due to extreme heat or cold. Meanwhile a 4 seasons porch allows year round usability through a heating and cooling system. Usually a mini-split system performs well for a 4 seasons sunroom.