In a bowl, combine the flour, salt, pepper and cayenne and mix well. Transfer the mixture to a plate. Heat the oil in a large pan until it just begins to smoke, then lower the heat slightly. Take the poblano rings 4 at a time, dip them in the flour, shake off any excess, then dip them in the buttermilk and back into the flour. Drop them into the hot oil and fry until lightly browned. Repeat with the rest of the rings and then drain on paper towels. Serve them warm.