Adding machines with several inputs

SO yeah hi, I'm making a mod with a TON of materials, and the more of them you can combine, the better they get.
To combine them, I've decided to create an Alloy Furnace, which takes up to 4 inputs + fuel, and outputs a stronger material
I have figure out how to create a basic furnace thanks to the tutorial form Pylo, but I cannot figure out how to give this furnace multiple inputs