Newbie needs help calculating field values

Hello, I am trying to create a fillable form where the user enters a dollar amount in three seperate fields: "cost A", "cost B", and "cost C".  I then have a fourth field ("subtotal") that adds "cost A", "cost B", and "cost C" together.  The fifth field is where I am struggling- I need the fifth field ("tax") to calculate the "subtotal" field by four percent or 0.04. I need the "tax" field to show a dollar amount, for example subtotal field equals $100.00, the tax field should calcualte 4% of the subtotal field and show $4.00.  The very last field is the "total amount due" which would be the sum of the subtotal and tax fields.  What is the best way to accomplish this?  Thank you in advance 🙂