Thread the Italian sausage, bell peppers, onion and three-quarters of the garlic cloves onto 8 skewers. Brush the kebabs with 3 tablespoons olive oil; season with salt and pepper.
Preheat an outdoor grill or grill pan to medium-high. Grill the kebabs, turning occasionally, until slightly charred, 12 to 15 minutes.
Meanwhile, in a large nonstick skillet, heat the remaining 1 tablespoon olive oil over medium-high heat. Add the remaining garlic cloves and cook for 1 minute. Add the spinach and cook, stirring, until warm, about 5 minutes; season with salt and pepper. Serve the kebabs with the spinach.