The S-4 was a modified S-2 partial pressure suit, no anti-G, chest bladder incorporated for ease of breathing. Also incorporated abdominal bladder for individuals who experienced abdominal difficulties, e.g. weak inguinal rings.

• Sears, William J, A Historical Review of Partial and Full Pressure Suits, . Accessed at: http://webpages.charter.net/mcelwain/PSUITS-Chronology.htm.