Coat grill rack with nonstick cooking spray. Preheat grill.
Stir together lemon juice, butter, 2 cloves garlic, and oregano; drizzle over fish and sprinkle with salt and pepper. Grill fish, covered, 4 to 6 minutes or until fish flakes with fork.
Heat oil in large skillet over medium-high heat; add spinach and remaining 2 cloves garlic; saute 5 minutes or until wilted. Serve fish over spinach. Top with feta and pine nuts, if desired.