Four Arms is a Tetramand (pronounced Tet-Rah-man-d) from Khoros (Khor-ose). It is the sixth alien to appear on the series. It was also Ben's most used alien form when he was his younger self, since that was Ben's favorite fighting style back then.
Possessing four arms, four eyes, armor-plated skin, and extremely dense musculature, Fourarms is rarely matched in raw strength. His strength is such that he can create shockwaves simply by smashing the ground or clapping all four hands together, dubbed the "Big Smack". Fourarms' muscular legs also allow him to leap several city blocks at a time.
Sometimes Fourarms' bulk is a disadvantage,since for it he is also not able to run very fast. Fourarms' massiveness makes him unable to hide anywhere or sneak behind anyone. He has been known to scare some people without meaning to and villains can get a hold of his leg easily because he is slow.
When Ben had a cold, Fourarms turned orange and had stinky hives. Strangely, when Ben tries to change into Fourarms, the Omnitrix seems to nearly always change into a different alien. This may be the AI of the Omnitrix which uses more strategy than Ben who simply likes smashing people.
Fourarms returns in Ben 10 Ultimate Alien. Fouarms now wears a golden X uniform, his eyes are green, the Omnitrix is now on his chest, and he wears tights and golden wrist bands. He also has a ponytail.
Tetramands are giants compared to most species, standing ten feet on average. The harsh wasteland of their home planet, Khoros, has turned the Tetramand race into physical powerhouses, a trait carried over to Fourarms.
Future Fourarms now has a short-sleeved shirt, which moved the Omnitrix symbol to the center of his chest. It also displays jagged spikes on the shoulders and arms along with two sharp fangs on the bottom corners of his mouth. In the future, his black pants completely covers his feet. A different Tetramand appears in the same appearance in Ben 10: Alien Force.