So many questions with this post. Smok is a fairly trusted brand. They've become one of the main name brands around the vaping industry, and I doubt they would put out a defective design. But alarm bells ring when you mention batteries getting so hot they "melt".
My first suspect would be the batteries themselves. with a multiple battery build like that, you need to make sure you buy four batteries at a time. Never mix old/new batteries. As you use lithium ion batteries, the ability to discharge slowly gets worse. A new battery, as a hypothetical example, might discharge down to 4 volts after an hour use. an older one might go down to 3.8 volts. If you have dissimilar or older/used batteries, the higher voltage battery will try to recharge the older battery as well as supply power to the atomizer. This can cause the battery to discharge quicker and get hotter. so, make sure you buy four batteries of the same type and at the same time, and keep those four together at all times.
Second, is make sure the batteries are high drain, and you get them from a reputable company. Even though seller claims they may be Sony batteries, and the wrapper looks like a Sony battery, there have been many cases where they buy a cheaper battery, re-wrap them to look like a Sony (or any other brand) battery, and sell them for twice the amount. The seller may not even know. Reputable sellers buy from trusted places and minimizes this.