Pokemon's Machamp, Ben 10's Fourarms, Undertale's Muffet, or Sleeps-Darkly's Alexis?
Machamp: A second pair of shoulders with arms and hands coming out from above or behind the main pair, where it logically makes sense for the back and pectoral muscles to interact with them.
Fourarms: An extra pair of shoulders with arms and hands coming out of the sides of the chest, beneath the main pair of arms, like an insect's extra limbs?
Muffet: The extra arms emerge from somewhere vaguely behind her around the shoulder or chest or back area so nobody has to think about it too hard and any artist drawing her in more detail can imagine it however they want.
Alexis: Or do you prefer when the pectorals and shoulders and breasts are doubled, like in https://e621.net/posts/5167406?q=four_arms+four_breasts this piece of art? I couldn't recall the names of any anime or cartoon characters whose four arms work that way so I searched for a character on this site for an example.