Hailing from the Hutt controlled moon of Nar Shaddaa, Z-N4 was once a simple security droid, providing protection to one of the many illicit clubs and casinos that can be found throughout the moon. That is, until he was stolen by one of the many gangs that also litter the moon's surface. They modified him, added on to his already combat oriented programming to make him into something more...deadly.
Which, turned out to be that particular gang's downfall. Working for the gang to take out their rivals one by one, Z-N4 slowly ended up gaining sentience for himself...and decided he was better off on his own. He destroyed his captors, his 'employers', and took off on his own path, making use of the skills he was given in his time with them to make a living for himself.
Standing at 6ft tall, Z-N4 is a more or less humanoid looking droid. Two arms, two legs, though with only three fingers on each hand. His plates are coloured a dark blue, though they're somewhat weathered and blasterscored from time and the dangers of his profession. He could get the superficial damage fixed...though he feels it adds to his intimidation factor.