Swiftwicks Pursuit four is the ideal sock for both you r cycling and trail running needs. Using a technical fiber of Merino Wool and an upgraded heel and toe pocket to give you that snug fit. the balance of the weave in the Merino will give you both warmth in the winter and early spring but cooling as well int he midst of a hot summer month. the four stands for the below calf height of the sock to give you that extra support for your running and cycling outings this spring and summer.