In order for a dragon to have four legs AND a pair of wings, and in order for characters like Goro from Mortal Kombat or Four Arms from Ben 10 to have two legs and four arms, they would all need to have a third girdle.
Okay, so what the hell is a girdle?
A girdle is the part of the skeleton where the limbs attach to the torso. Vertebrates generally have two; the pelvic girdle (hips) and the pectoral girdle (collar bones and shoulder blades). The reason for this is because vertebrates evolved from creatures that had two set of limbs, the early lobe finned fish.
Over the eons, these fins adapted into analogous structures to fill different niches. So, there can’t be any six-limbed vertebrates because there’s nowhere for the limbs to attach. There would need to be a third girdle in between the first two to make room for a third pair of limbs, be they extra arms or wings, or whatever.
It is relatively easy to take an existing body part and adapt it into something else; legs are analogous to wings are analogous to fins. You can tell what all the bones used to be, and that they’ve simply changed orientation and shape on an evolutionary timescale.
But it is very, very, very difficult to just create a new body part out of nothing. It would be impossible to evolve an extra pair of legs or arms or wings because there is no analogous structure to make it out of. You can lose a body part, like whales lost their hindlegs or how snakes lost all four, but you can’t gain new ones apropos of nothing.
Real dragons would be more like pterosaurs, with two legs and a pair of wings (in mythology, creatures like this are called wyverns).
