Dont know if they are still available, but years ago when we did this, it was advisable to use something called a Color Key. Color key was a product that I believe was from 3M that had each colors individual image exposed on its own sheet of clear acetate. All four colors were registered to each other and mounted on a sheet of paper similar to what the job was to run on. With this color key you could seperate each of the colors individually, or combine them in stages as the job required.
They truly were the good ole days for the printing industry ,where a company could actually make money running 4 over 4, on a single color press. If you do the math thats 8 passes through the press not including varnish.