Let's see: you start with 84 quarters.
Now you change every 2nd quarter with a dime; so you now have 42 quarters (those in odd-numbered locations) and 42 dimes (those in even numbered locations).
When you change every third coin with a nickel, half of them will come from odd-numbered locations and half will come from even-numbered locations. Since 84 divided by 3 is 28, 14 nickels will be in odd-numbered locations and 14 will be in even numbered locations. You now have 28 quarters and 14 nickels in odd-numbered locations and 28 dimes and 14 nickels in even-numbered locations.
Changing every fourth coin with a penny, will place 21 pennies, all in even-numbered locations; 14 in locations that have dimes and 7 in locations that have nickels.
In odd-numbered locations, you have 28 quarters and 14 nickels; in even-numbered locations, you have 14 dimes, 14 pennies, 7 nickels and 7 pennies.
Total: 28 quarters, 14 dimes, 21 nickels, and 21 pennies