Step 1: Cook the Pasta

1. Bring a large pot of salted water to a rolling boil.
2. Add fettuccine or spaghetti, stirring occasionally to prevent sticking.
3. Cook until al dente, following package instructions.
4. Reserve 1 cup of pasta water, then drain the pasta and set aside.

Step 2: Cook the Chicken and Bacon

1. Heat a large skillet over medium-high heat.
2. Add the chopped bacon and cook until crispy, about 5 minutes. Remove and set aside.
3. In the same skillet, add cubed chicken and season with salt and black pepper.
4. Cook until golden brown and fully cooked, about 5-7 minutes per side. Remove and set aside.

Step 3: Make the Four-Cheese Sauce

1. Reduce heat to low and add minced garlic, cooking for 30 seconds.
2. In a separate bowl, whisk together heavy cream, egg yolks, and black pepper.
3. Slowly pour into the skillet, stirring constantly.
4. Add Parmesan, Pecorino Romano, Mozzarella, and Gruyère, stirring until melted.

Step 4: Combine Everything

1. Return the cooked pasta, chicken, and bacon to the skillet.
2. Toss everything together, ensuring the pasta is coated in the cheesy sauce.
3. If the sauce is too thick, add reserved pasta water in small amounts.

Step 5: Final Touches & Serving

1. Remove from heat and stir in butter for extra richness.
2. Garnish with chopped parsley and extra Parmesan cheese.
3. Serve immediately while warm and creamy.