The mystic forge is a special crafting station that can be used by all characters. Recipes in the mystic forge always require four ingredients. Recipes can be discovered in a similar fashion to crafting professions, but there is no tab to record them. Generic recipes will show all other equipment of the same rarity as valid in terms of the discovery.
Generic recipes typically require four pieces of equipment of the same rarity and will create a random piece of equipment of that rarity whose level is 5-12 levels higher with a chance that the item will be promoted in rarity.. If four pieces of equipment of the same type and rarity are used, the equipment received will be of the same type in addition to normal function. This serves as a way to obtain high level versions of a certain piece of equipment. The forge can also be used to transmute crafting materials, dyes, and blueprints among other items.
Recipes unique to the mystic forge are used to create specific items such as mystic or legendary weapons. These items often require mystic coins, high level items, and large resources.
You can help with Research into Forge Recipes here.