Place three pitted and peeled avocados in a large bowl along with the lime juice, garlic, serrano pepper, cumin, and salt. Mash with a fork until the mixture is only slightly lumpy. Dice the last avocado into small pieces and add it to the bowl along with the remaining ingredients. Gently stir the guacamole to combine. Keep chilled until ready to serve.
*to serve the guacamole in a pumpkin, simply carve a small pumpkin and remove it's seeds and insides. Place the pumpkin on a serving dish and pour the guacamole in front of the pumpkin's mouth, pushing a little guacamole up to it's mouth so it looks like it's coming out of the mouth. Serve with chips on the side.