Cover work surface with a waterproof table cover. Cover this with plastic wrap or aluminum foil on which to build the volcano. Place bottle on table, lightly scrunch up the plastic grocery bags & place them around the bottle (do not cover the bottle opening), taping in place until you are pleased with the shape.
Thin the white glue with a little bit of water mixing it in the flat container. Don’t make it too runny. Throughly soak each coffee filter in the mix working with one at a time. Place it on the bagged volcano shape. Continue placing filters on the volcano, overlapping where necessary. Smooth out any air bubbles. One layer was enough. Let dry. Give the volcano a couple of coats of mod podge to add strength. Let dry between coats. Once dry, carefully peel away the plastic bags. The volcano is ready for paint.
Let the four year old go to town creating a volcano masterpiece!
To use the volcano: Place the paper mache volcano over the same bottle used to construct the form. Add your favorite kid-friendly explosive mix (baking soda, vinegar, & food coloring) & watch the eruption with the same glee as a kid!