I think fabrials probably could be called the 4th Metallic Art at this point. But I guess it depends on where you think being a Metallic Art ends, and being a way of using investiture that is entirely governed by the properties of the same set of metals begins. I'm not sure what the difference is at this point given Hemalurgy is a metallic art that's not reliant on Preservation's investiture, and they all use metals as keys for manipulating investiture sources.
I suspect that the nature of awakened metal on Nalthis is probably influenced based on which of the 16 metals are used as well. Is that maybe one reason - beyond commands -why Nightblood and Vivenna's sword are different?
I also wonder if using Awakening or Feruchemy on fabrial metal would be influential. Could you create conscious metal in the fabrial that would self-govern the way these primitive surges are manipulated?