The NS-4's were the earlier model of advanced humanoid droids before the release of the NS-5's, produced by US Robotics (USR for short).
As with all USR Robots, they were "3 Laws Safe", meaning they adhered to the Three Laws of Robotics, which were hard wired in the integral system of each machine.
They were customised randomly with different colours throughout the years as owners decided individually how they wished their personal robots to appear.
When the new NS-5 model was brought out by USR, people were encouraged to trade in the NS-4s in exchange and large numbers of them were sent into storage.
Later, when VIKI engineered the NS-5 attempted takeover, a pre-emptive strike was carried out against the NS-4s in storage at the Lake Michigan facility by a large task force of NS-5s. The more efficient, stronger and more fast and flexible NS-5s managed to destroy the NS-4s completely, despite some resistance. The reason VIKI had the NS-5s carry out this attack was due to the fact that the NS-4s were not linked wirelessly to VIKI at the USR Headquarters, and so therefore would have attempted to protect the humans from the NS-5 revolt under the First Law of Robotics.
Indeed, when a group of NS-5s saw Detective Del Spooner observing in horror their attack on the NS-4s and gave chase with intentions to kill him, a number of NS-4s leapt from their storage in order to protect Spooner, observing "Human in Danger". Although they were rapidly destroyed by the NS-5s, their obedience to the First Law enabled Spooner to escape and so they were ultimately successful in fulfilling the Law.