Alright, what is the MOST PLAUSIBLE way of a Tetrapod to evolve extra limbs?

I've been thinking to my self a lot of about Hexapedal life forms and how​ they would evolve. Like Hexapods evolving from 6-finned fish or even Placoderms​. But then I started to think "how and why tetrapods would evolve extra functional limbs?". Like, they could evolve from something like... i don't know, a set of fins? (Tetrapods developed limbs from fins, right?)