The article presents the results of breeding work (2004–2018), aimed to create a highly winter hardiness hay-pasture variety of alsike clover (Trifolium hybridum L.) KB-4M, which forms 9–11 t/ha of dry matter in 2–3 mowings, with a seed productivity 130–150 kg/ha. The breeding population KB-4M was created in the Federal Williams Research Center of Forage Production and Agroecology. This breeding population is a complex hybrid population created as a result of free-confined pollination of tetraploid genotypes. The genotypes of 'Krasavik' variety are female parents. 'Krasavik' variety was created in the Research and Practical Center of National Academy of Sciences of the Republic of Belarus for Arable Farming. Pollinating varieties: 'Alpo' (Norway), 'Högsta' (Sweden), 'Eutetra' (Germany). The breeding population KB-4М is mid-ripening. Period from spring growth till first cutting is 37–47 days, till seeds ripening — 112–120 days. The winter hardiness is quite high (90.0–99.8%). During the period of competitive variety testing (2014–2016) the breeding population KB-4M has demonstrated an advantage over the standard 'Pervenets' in terms of the main economically valuable properties. For the three testing cycles have been demonstrated the following characteristics of yielding ability: green mass — 48.8–64.3 t/ha (9.9–24.6% more than the standard), hay — 9.14–12.37 t/ha (18.2–23.3% more than the standard), seeds — 130–150 kg/ha (18.2–22.7% more than the standard). The nutrient value: crude protein, depending on the cut, 169–223 g/kg dry matter, crude fiber — 139–275 g/kg. In 2020, the breeding population KB-4M was transferred to the state variety testing. This breeding population recommended for hayfields and pastures in all regions of alsike clover cultivation, especially for heavy, loamy, wet soils with high acidity (pH less than 5.5) and thin humus-layer, as well as for: grassing and land resting, cultivation of bog soils and peat bogs, establishment of melliferous crops.