Add water to a large skillet, filling two-thirds full; bring to a boil. Reduce heat; simmer. Break eggs into each of 4 (8-ounce) custard cups coated with cooking spray. Place custard cups in simmering water in pan. Cover pan; cook 4 minutes or until yolk is just set or until desired degree of doneness. Remove custard cups from water.
Combine pesto and 4 teaspoons water. Spread 1 teaspoon pesto mixture on cut side of each of 4 English muffin halves. Divide prosciutto evenly among 4 English muffin halves.
Run a small rubber spatula around outside edge of each custard cup; slide 1 egg onto each muffin half. Sprinkle eggs evenly with pepper, and drizzle each egg with 1 teaspoon remaining pesto mixture. Top with remaining muffin halves, cut side down. Serve immediately.