After 16 legendary crafting attempts eventually managed to load/reload to craft Johnny's Jacket to 4 Mod Slots. Forgive me but IMHO this is Johnny's jacket, and by default once you can craft the legendary version, it should be 4 slots minimum.
The mods currently occupying the slots came with the craft just if you were curious.
The mods currently occupying the slots came with the craft just if you were curious.