The SG-4 blaster rifle was developed by the Imperial Department of Military Research due to the fact that a regular blaster's range was reduced in water. The SG-4 was a combination weapon that fired normal blaster bolts out of water and miniature harpoon rounds underwater.
The Merr-Sonn Munitions SG-4 blaster carbine was generally promoted as a security and police tactical response weapon. It served as a light duty weapon on some mobile planetary defense units also. Shortly before the formation of the actual Rebel Alliance, agents who would go on to become members of the Alliance posed as law enforcement officials and successfully purchased over 200,000 of these carbines. Some of these weapons were later used by Rebels, with others being scattered over several secret caches. One such cache, containing over 2,000 of these weapons, was discovered by Imperial Security Bureau agents shortly before the outbreak of anti-Imperial hostilities. Presumably, these weapons were either confiscated by the ISB or sold under the table on the black market for an extra profit.