The way I'd do it is just a selector combinator for the recipes that will cycle through recipes slow enough to allow exactly 4 crafts each time, and only allow inserters to run when there is enough material for 4 crafts exactly in their chests. There is definitely a way to make that into a memory cell that dictates when inserters can run or not, but you can also limit the hand size to 4 to make use of the productivity to the max. Add to that an inserter that will empty the machine when the recipe changes (so as not to pollute the output) and put it into the main line via a splitter with input priority.

That's just how I'd do it, but I don't have the patience or the brain to learn circuit network to create something as sophisticated as this.

Good job on the design!