Parallel evolution. The humanoid form is one that really lends itself to sapience. Dextrous hands with opposable thumbs are great for manipulating the environment, so any 4-limbed species heading for intelligence would as a matter of course develop an upright posture to accomodate them.>but why 4 limbs on other planets???
Because it's an efficient design for moving on land. In a world that hasn't yet developed the specialized musculatures required for bipedalism, 4 legs is the lowest possible number of legs an organism can have while remaining stable, and fewer bodyparts means lower energy requirements, and that's a big part of why many-limbed arthropods don't dominate the planet.>but what if the alien planet is different in ways you don't even know???
Aliens from such a world probably wouldn't be interested in Earth. When we look for planets that could have life, we look for Earth-like planets, and Earth-like planets would necessarily have life that would make sense on Earth. It's probably the same for aliens. An intelligent species from Jupiter would look for life in gas giants, not in tiny balls of carbon and water that wouldn't be capable of supporting complex life as they know it.