Turning home made chicken stock into broth
If you are just going to season your stock (according to the formula "broth = seasoned stock"), simple math will suffice:  4 cups = 32 * 1/8 cups -> 32 of your cubes.
But if you are planning on using the stock and re-boil it wit fresh meat and vegs you should consider using more stock, because you will have to calculate for evaporation. Depending on your habits (do you cover your pot, how long are you boiling it) I'm guesstimating between 10% and 20% extra. If you end up with too little, you should be fine with a dash or two of water without compromising the results.