W4 line 4a question

If I have $20,000 in Interest income every year and I want to add that to my W4 so it get taxed, do I add the total amount $20,000? Does my companies payroll divide out the total amount by 26 (the number of paychecks per year) and use that amount per pay check? Or do i have to break it out $769 and add that to line 4a?