The T-400 was invented during initial tests to mimic human autonomy. The idea was to create a Hunter Killer that had the general size, shape, range of motion and mobility of a human being. The hyper-alloy construction and armor of later series was not perfected during this time, so the 400 Series' construction was made from cheap and not particularly durable materials, that were easy and fast to produce and put into the field. Due to these reasons, Skynet kept producing the 400 Series en masse as expendable soldiers even after newer Humanoid Hunter Killer series were created, such as the T-500 or T-600. The 400 Series served as the main ground-troops shortly after their creation, and some were used as guards at different skynet installations, factories and internment camps, where human prisoners were held.
At the time of its creation, the T-400 was an impressive and even menacing foot-soldier, standing roughly about 7 feet tall and weighed a quarter of a ton. It was physically strong and could lift about 1000 lbs of weight. It was known to be able to ( and sometimes even fond of ) break through concrete and reinforced walls. It could easily beat a human in close combat, unless the human enemy was intelligent and cunning enough to use it's weaknesses against it.
The T-400 series, unlike later Terminator series, had several areas were its wiring was exposed, which became a huge weakness of the T-400 series since in time, Resistance fighters learned to always aim for these spots. It also did not help that the T-400 had only one big, red optical sensor that glowed red. This optical sensor with its bright glow were a dead giveaway to any Resistance snipers out in the field, who then often proceeded to pick off the T-400's afar.
The T-400 series were one of the very first Hunter Killer series that utilized true artificial intelligence, and thus, had a very primitive CPU compared to later Hunter Killer and Terminator series. It had just about enough intelligence to be able to walk, shoot straight, follow simple orders, hunt and eliminate a target and was able to tell the difference between allies and enemies. Its limited intelligence impaired the T-400 in many ways, one of them being incapable to adapt to constant changes in the field. This, of course, gave the T-400 enormous difficulty at battlefield improvisation, and in time became predictable and easily outsmarted by Resistance fighters. Their mentality could be compared to either an ape or a very slow-witted child. 
Ultimate Fate of the T-400
The T-400 was eventually deemed obsolete by its creator as newer Humanoid Hunter Killers and Infiltrators were developed. Their cheap, obsolete armor was not enough to protect from the more modern weapons such as Plasma rifles, and even rusted. Instead, most of them became clean-up squads, who were tasked with terminating any survivors from attacks on human camps that were performed by more successful Humanoid units or Infiltrator units. Some are also still seen participating in Demolition Squads, who are tasked with either removing or setting up explosives. 
- Video games
- The Terminator: Dawn of Fate (First appearance)
- Terminator Hunt (Mentioned only)
- Terminator Salvation: Cold War (Mentioned only)
- Terminator Salvation: Trial by Fire (Mentioned only)