The Series 4000 is the model of Mechanoid of which Kryten and Able is. Developed by DivaDroid International was much better received than the Series 3000 mechanoid. The Series 4000 model looks like robots whereas the Series 3000 model looked too much like real humans - which caused a disturbing reaction among consumers. "RD: Out of Time"
With the Series 4000 mechanoid the head is shaped in almost a cube and is, as with the arms and other parts of the body, detachable and replaceable. The Series 4000 mechanoid also has a groinal attachment which is a long tube attached at the groin of the mechanoid it can have a vacuum cleaner, egg whisk and hedge trimmer attached to it. They often have numerous Spare Heads and can detach various body parts, including eyes and hands. Their brains were part organic ("RD: D.N.A."), based on that of John Warburton, and could use and get addicted to Outrozone. ("RD: Beyond a Joke") They could eat certain foods ("RD: The Last Day") and were programmed in the use of Anti-Matter Chopsticks. ("RD: Legion") Their sanitation software was installed at "Toilet University". ("RD: Epideme") Their chest monitors could be used as displays. ("RD: Tikka to Ride") Ace Rimmer said the Series 4000 was "the salt of the Space Corps". "RD: Dimension Jump"
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".
The Nega-Drive
In the Series VII episode "Beyond a Joke", Kryten meets another Series 4000 mechanoid Able and learns the terrible secret of his design. It is revealed that the entire 4000 series of mechanoids was 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 blow up. It seems likely that the only reason DivaDroid developed the model was due to the reception of the Series 3000.