Preheat oven to 400°F. Spray rimmed large baking sheet with cooking spray.
Spread green beans into single layer on baking sheet. Spray with cooking spray. Sprinkle salt, pepper, onion powder and cayenne evenly over green beans,
Place baking sheet in center rack of oven and roast for 20 minutes. Use spatula to turn green beans over and roast another 20 minutes until starting to dry and ends become browned and crisp.
Remove pan from oven and serve warm with dipping sauce