Codes are vital in distinguishing different versions. Germans of all people have the tendency to document and number thoroughly. Unbelievable but proven beyond doubt ca January 1943 some 251 variants were renumbered.
The KSTN 1123(gp) dated November 1st 1941 indicates (among others) the /3 as the "m Schütz Pz Wg (I.G.)" and the /4 as the "m Schütz Pz Wg (I.G. ammo)"
The D-660/7 ( cover, Indexpage) loading manual, August 13th 1942, lists the /3 as an artillery towing vehicle. The /4 is the ammo version.
The KSTN 1123a(gp) dated March 1st 1943 lists 2 /4's as the artillery towing vehicle and 1 /4 as an ammo vehicle.
The D-600 of July 6th of 1943 lists ( Part 1, Part 2)the /3 as a radio vehicle and the /4 as the artillery towing vehicle.
Many thanks to Dr. Leo Niehorster for his superb information and keen eye for details.