Under Armour Women's ColdGear Base 4.0 Half Zip Long Sleeve Base Layer Shirt - UA Base is a system of baselayers, numbered from 2.0 – 4.0, that takes you from cold to absolutely freezing temps. It uses a special negative grid structure to trap heat without adding bulk. Think long underwear, reengineered for athletes performing at their peak. UA Base 4.0 is built for extreme cold weather and lower activity levels.