What is the difference between a 3 season sunroom and a 4 season sunroom (year round sunroom). The obvious answer - one season! A 3 season is built to be used in the warmer months of the year, which can vary from a span of 5 to 8 months (if we’re lucky!). A 4 season sunroom is constructed to allow the sunroom to be used comfortably year round. There are three primary differences in the construction of these rooms: glass, insulation and a HVAC unit.