Patrol droids have been in use for as long as droids have existed. The early models were crude, with a low sensor range and a limited radius of action as they were dependent on remote access from a control facility. Later models were equipped with their own AI and would utilise a much more advanced CPU. Their action radius increased as did exposure to hostile encounters. This inevitably led to more advanced droid types. The Mark IV is a more recent development by engineers of the Galactic Empire, and its main purpose is to ferret out local resistance cells, and to scout for potential targets during a concentrated attack.
Individual patrol droids were configured differently and equipped for the special needs a specific mission would demand. The Mark IV was designed for sentry purposes only and thereby lacks any weaponry or any noteworthy armour. The space left available by lack of weaponry was used to improve the droid's sensors and mobility. The Mark IV's small size combined with its enhanced sensors and mobility allows the droid to be capable of dodging a direct attack by an aggressor and escaping. Unlike other Imperial droids, it does not possess a self-destruct ordinance and does not store any data. Direct transmissions of surveillance data are sent in a coded stream to the base of operations.
An additional benefit to the droid's rather small size is that it can easily penetrate hostile lines and inspect the terrain or scrutinise the opposing forces. This special make-up is equipped with an ionic shield and can sustain minimal damage. The limiting factor of the droid's range of operation is the lack of any means of defence. The droid can try to flee if discovered, but would have trouble against a droid specially equipped for anti-droid missions (such as an HK-unit).
Imperial scouts and forward parties use the Mark IV extensively in their reconnaissance missions. It allows, if undetected, for an aerial assessment of the theatre and requires only minimal supervisory manpower. Since their usefulness is unquestioned, the Empire produces them in large quantities at reasonable unit costs. Due to this process raw material usage has been reduced to an absolute minimum.
The Mark IV operates basically in every environment and in a variety of conditions. The only exceptions are high pressure areas like gas giants and the deep sea. In addition, the droid's propulsion systems fail on volcanic terrain due to the immense heat they would absorb during normal operation in such high temperature environments.