The AS4 is our medium-soft mattress. Itâs great for side sleepers, and anyone who likes more cushioning while theyâre in bed. We combine our proprietary, cooling Bio-Pur foam and Affinity with SMT comfort foam layers to create a soft but responsive feel that relieves pressure points and makes you feel like youâre floating. Plus the high-performance Celliant cover is made with thermoreactive yarn to safely turn body heat into infrared.