I can tell you that the SST-4 had nothing to do with the sinking of ARA San Juan. In 1982 the MoD tried to obtain data on its capabilities from its manufactures in Germany. They politely declined. More later.
When ARA Santa Fe was damaged in South Georgia and before it was subsequently scuttled a number of its SST-4 fish were unshipped and sent to the UK for evaluation. The results made the Mk24 Tigerfish look good.
When Argentina bought its three subs in the late 70s, the SST-4 was part of the deal.
In regards to what happened, I can only suggest a theory.
She took on water through a Snorkel failure. This caused battery damage which led to a power failure while submerged. With 2 tons of sea water on board and despite blowing main vents, without electrical power the outcome was inevitable. Without power, the bilge pumps could not clear the influx and gravity did the rest.