The related question about possible weapons for four-armed humans provoked me to ask one of my own.
I have a technologically advanced (to somewhere of the Mass Effect levels of technology) alien species that have four arms, that are main characters in my webcomic:
And that question about weapons had me thinking, what four arms could mean for things like tool design and such? Is there any unique inventions that can benefit from having one additional pair of limbs to operate, and\or would an additional pair of limb provoke some distinct design choices?
There are some anatomical considerations and limitations: The upper pair of arms are basically the complete equivalent to human hands in both dexterity and range of motions. Lower part of hands are biomechanically equivalent to the upper part, except that they have no clavicles, cannot be raised sideways higher than 60° due to differences in the shoulder joint muscle attachments, and in general are weaker than the upper limbs. On the plus side, due to no clavicles they have wider available sliding motion range, being able to extend further to the back or closer to the chest of the alien (similarly to how real life people born with CCD can make their shoulders almost touch). They can control different sets of arms to perform different tasks, but to a very limited degree (Somewhat better than the classic "rub your tummy while patting your head" challenge - it's doable but requires concentration on the task and some amount of effort). Like humans they have dominant hands, with the lower dominant hand always being on the opposite site of the upper dominant hand due to crosslinking in the nervous system. Ambidexterity (...or would it be "quadridexterity"?) is rarer than in humans but not unheard of.