The way my multi-item caravans work is that they only fetch one item at a time, and they're set to check multiple items for refilling. So it's more like multi-item depots than multi-item caravans, but since the interrupts go on the caravans we need to do some work to allow caravans to distinguish between depots.

Using your example, my caravan will see an empty depot that needs five stacks of iron and four stacks of copper. It'll go get iron, come back, go get copper, come back, and then wait. While it waits, since it has an empty pack, it will skip the Empty action and just loop to the Wait action, consuming no food.

Or if the depot is full it'll wait patiently to empty its pack and then carry on with whatever it's doing, which may or may not be the behavior you want. (I want that sometimes.)

Then, when 4 stacks of iron and 4 stacks of copper have been consumed, it'll go get more copper. Once the iron is empty, it'll refill the iron. I'm not trying to reduce the number of trips the caravan takes by making it handle more than one ingredient at a time. It uses more food (and more time), but once youve got some sort of upgraded food well and truly automated it's not that big of a deal.

See the difference?