You want to CO 4 less stitches (because those 4 stitches you loose when you sew it up). For example you loose two stitches when you sew up the right side of the dress, and two stitches when you dew up the left side, so a total of 4.
You want to keep the decreases in the same position-so put in an extra stitch marker exactly half the distance from your marker that marks the beginning of the round in order to mark the invisible sides where the front and back "come together" (use a different color maker so you can distinguish between the two). Using the markers this way will help to see that your knitting is still divided into two parts- the front and the back, and that you still follow the pattern for each of the respective parts.
Hope that helps.