SS-4 Sandal is the NATO's version name (nickname) for a Soviet's Medium Range Ballistic Missiles (MRBM) that originally had a name or registration number R-12 in Uni Soviet. No information founded why someone in NATO named it with "sandal?" This thing didn't look like sandal (footwear) at all or maybe its named by the name of a place? or that person from NATO didn't had any idea and looking for the sandals that he/she wearing and got name for it :)

SS is the NATO's code for Surface to Surface missile, and the nickname also has a beginning letter "s," for example SS-1 missile is nicknamed as "Scud." sources: (1) (2)

This missile could launch a three-megaton nuclear warhead to an estimated range of 1,020 nautical miles (2000 km). Soviet using SS-4 Sandal as one of several types of warhead that deployed on Cuba soil, when Soviet Union had confronted with United Stated of America in the summer of 1962 also know as the Cuban Missile Crisis.

This missile has 22 meters length and built in 1955 when first tested at 1957, there are several development of this missile type, before eliminated after the end of cold war at 1990.

You can see this missiles displayed at "Stalin Line" Museum, Belarus, former Soviet Union country.
Gallery-1 gallery-2

Sources:
http://www.fas.org/nuke/guide/russia/theater/r-12.htm
http://www.globalsecurity.org/wmd/world/russia/r-12.htm
http://www.russianspaceweb.com/r12.html
http://svsm.org/gallery/r-12
http://www.babinets.com/belarus/belba202.html
http://www.squidoo.com/cubanmissilecrisis/
http://en.wikipedia.org/wiki/NATO_reporting_name
http://www.designation-systems.net/non-us/soviet.html
http://en.valka.cz/viewtopic.php/title/SOV-8K63-R-12-SS-4-Sandal/t/20537

 1st-sandal