Temperate climates may have four seasons but wardrobes benefit from three, despite the clothing business's division of the year into two. Clothing's autumn/winter and spring/summer seasons do not quite do the trick when it comes to tailored clothing, for jackets wear comfortably within a range of perhaps 25 degrees Fahrenheit (15 degrees C).
Men's jackets are worn without additional outerwear between freezing and perhaps 90 degrees F (32C). Since that range is considerably greater then may be accommodated by two jacket weights, there are going to be temperatures at which a man is going to be uncomfortable with a wardrobe built for only two seasons.
Many men choose to span this temperature gap with a topcoat. This is practical. Wardrobe building is easiest when a man acquires mid-weight items first, and then expands to cover his summer requirements with light-weight stuff. He dons his topcoat when the mid-weight cloth is no longer warm enough, and a single coat may substitute for several heavy suits or odd jackets. The challenge with this approach is that walking any distance outdoors on very cold days means the lower legs can become uncomfortably cold. The solution of course is to wear heavier trousers, which leads the wardrobe into the third season, that of heavy cloth.
For climates with perhaps two very cold months of the year, four heavy suits (14-16 ounce/420-480 gram cloth is about as warm as most men can wear in heated offices) are likely to cover the temperature gap. In addition to more flexible temperature control, heavier suits mean that on many days the topcoat can be left at home, simplifying one's personal logistics considerably.
A first heavy suit should probably wait until a man has at least six suits for summer and six more in mid-weight cloth. But that is the time to begin considering how to bridge the termperature gap.