Remove peel and pit from all avocados. Use either a large bowl and fork or a food processor (do not over-puree) to mash three of the avocados with all of the sour cream until creamy. Fold in Serrano, onion, and cilantro. Dice remaining avocado into 1/4" cubes. Fold avocado, tomatoes, and lime juice in to mixture. Add salt to taste. Serve immediately or refrigerate for 30 minutes.