If we go full science based then we'd all be using Wyverns not true dragons because a six limbed veterbrate...is kinda impossible?
Well, it depends on what you do with it.
Most of a creatures' weight is in water weight, so yeet the blood and you yeet about 60% of the weight.
That frees up enough weight so that you could feasibly have a decent enough wingspan for flight, but it also means they have a branched respiratory system/digestive system with essentially a paper-passer way of punting energy around the body. Not ideal, but I can work with it. So, as it is, these guys would have had to evolved from f
latworms,
nematodes, or
cnidarians. So they came out of the water and are made of jelly. Cool. About seventeen million years worth of evolution later, and we have a snake thing with a skeletal structure. Fifteen more mutations and we have limbs. Boom.
To explain the six limbs, toss 'em in a cavern full of massive stretches of space, where walking would take more energy than it's worth, but walking is also needed. There you go. Four legs and a pair of glidy wing things.
Reinforce the bones with something (carbon would work best) with triangle supports and you have strong, lightweight bones, no bloodweight, and six limbs.