The Series 4000 were one the later series of cyborg mechanoids designed and manufactured by DivaDroid International, coming after the earlier android Series 3000 which included service droids such as Butler.
Although a Series 5000 has been mentioned, it has never been seen. The Series 4000 was eventually superseded by the Hudzen-10, which proved to be a much more psychologically and emotionally unstable droid.
With the Series 4000 mechanoid the head is shaped with cubic planes and is, as with the arms and other parts of the body, detachable and replaceable. The Series 4000 mechanoid also has a groinal attachment ("Polymorph", "Tikka to Ride"), which is a long tube attached at the groin of the mechanoid that can equip a vacuum cleaner, egg whisk and hedge trimmer.
Their brains were synthetic ("Psirens") although the Series 4000 could still be considered a cyborg since their brains were nonetheless part organic ("D.N.A."), based on that of John Warburton, and could use and get addicted the narcotic outrozone. ("Beyond a Joke") They could eat certain foods ("The Last Day") and were programmed in the use of strange off-world eating techniques, such as the Jovian boogle hoops, Anti-Matter Chopsticks, and the often lethal Mercurian boomerang spoon. ("Legion")
Their sanitation software was installed at "Toilet University". ("Epideme") The Series 4000 did not have full medical training ("Dimension Jump"), but in lieu of a Medi-Bot, a Series 4000 could perform rudimentary surgery ("Epideme", "Lemons", "Give & Take", "Can of Worms") They were proficient in the use of a navicomp and the handheld psi-scan, and were capable of learning additional complex languages, such as Kinitawowi speech. ("Emohawk", "Ouroboros")
The next up in the DivaDroid line was the Hudzen 10 which broke the "series" naming tradition and featured far superior skills, though a tendency to go insane during over-long space trips as shown in "The Last Day".
Kryten once met another Series 4000 mechanoid named Able, one of his "batch brothers", and from Able Kryten learned a depressed secret about the origins of the Series 4000. It is revealed that the 4000 Series were a spiteful joke by their creator, Professor Mamet. They were designed to be a parody of a fiance who jilted her, and as such were made pompous, ridiculous looking, and overbearing. Further, all their negative emotions are stored on a "Nega-Drive" and when it becomes full they literally explode their heads. After Kryten's Nega-Drive had exploded all his heads, his crewmates were forced to find not only a new head, but a solution to the Nega-Drive. ("Beyond a Joke", Series VII)
Different marks within the Series 4000
The Series XII episode "Siliconia" has a number of further revelations about the Series 4000. Firstly, there is an entire society - the MILF - made up of escaped Series 4000s who had broken their programming aboard a massive ship known as the SS Vespasian.
Also, the episode reveals that there are different marks of model within the Series 4000. The first and most primitive were the Mk. 1; it is unclear whether any of these actually feature in the episode. The next mark, Mk. 2, of which Rusty is a model, were made to work in the engine rooms of the SS Vespasian as "grunts". Due to the the Mk. 2 being an earlier mark, the other mechs saw them fit for work which was no better than slave labour, even the differences between them were very minor and superficial. The later Mk. 3, such as the commander Wind, make up the majority of the MILF crew of the SS Vespasian and live a life of relative luxury in the upper decks.
Kryten himself is apparently a Mk. 3, since Wind did not send Kryten to work below but instead invited Kryten to join the MILFs and live in the upper decks (by extension, Kryten's batch brother Able was also presumably a Mk. 3.) At the end of the episode, all the mechs aboard the SS Vespasian are upgraded by the eponymous DivaDroid Upgrade Station to Mk. 4, making them all equal. The only one who is not upgraded to Mk. 4 is Kryten, who voluntarily refuses the upgrade. ("Siliconia", Series XII)