Your mixing cup with have a number of sections. These sections will be the different ratios and then there is usually one or two with just a "ruler" with equal parts and then also parts in ounces.
In the sections of ratios you will have a few columns. Find the section that is 4:2:1. In that section there look at the first column and you will see numbers going up 1,2,3,4 and so on. The first column is your "4" where you will add your paint, clear of primer. The second column is the "2" in your 4:2:1 ratio and that is where you will add your reducer. The last column is your "1" and that is where you will add your hardener.
Ok, look that last column and find the number that will give you the amount of mixed paint, primer or clear that you want.
Let's say it is a half a pint and you are mixing in a pint mixing cup. We will say that the middle of the cup is at the number 5 in the hardener column. That is where you finished mix is going to be.
So, you will pour your paint, primer, or clear up the number 5 in the first column (this is in the 4:2:1 ratio section remember). So, you have the paint,primer, clear up to that number 5 in the first column.
You now add the reducer up the number 5 that is in the second column. That number 5 is about one half more than the first column you will notice. It is two "parts" more than the first column at the number 5. You are now with the paint,primer, and clear, along with the reducer up to the number 5 in the second column.
Now you add your hardener up the number 5 in the last of the three columns in your 4:2:1 ratio section and that is it.
Mix it up and you are ready to go.
Remember "parts" are simply equal parts. Be it table spoons or 55 gallon drums, just equal parts.
You could, if you wished, mix 4 tablespoons to 2 tablespoons to 1 tablespoon.
Or mix four 55 gallon drums to two 55 gallon drums to one 55 gallon drum.
They both would do the exact same thing. Just the same as the 4:2:1 in your mixing cup.