In a large saucepan, melt the 4 tablespoons of butter over medium heat. This will serve as the base for your dip.
Once the butter is melted, whisk in the 4 tablespoons of flour. Continue whisking for about 1-2 minutes until it forms a smooth paste.
Gradually pour in the 3 cups of milk while whisking continuously. Keep stirring until the mixture thickens and starts to bubble gently.
Lower the heat and add the 2 cups of Velveeta white queso and 2 cups of Velveeta pepper jack cheese, stirring until the cheeses are completely melted and the mixture is smooth.
Stir in the 1/2 cup of diced red peppers, 2 tablespoons of diced jalapeños, and the 4 ounces of thawed and drained spinach. Mix everything together until well combined.
Transfer the dip to a baking dish or serve it straight from the saucepan. Serve warm with tortilla chips, pita bread, or fresh veggies for dipping.
