The first step in solving the equation \( 20 = 6x - 4 \) is to add 4 to both sides.
This will help isolate the term with \( x \) on one side. Here's how it would look:
\[ 20 + 4 = 6x - 4 + 4 \]
Which simplifies to:
\[ 24 = 6x \]
So the correct response is: Add 4 to both sides.