Which method of declaring a private class method feels most idiomatic now?

private_class_method.rb

# Names of classes and methods have been changed to protect the innocent. Namely
# my sweet, innocent, cherubic, and hopefully continuing, employment.
class MessageTwiddler

  # Okay, so: say I want to make a class method private. What's the best idiom
  # for doing this in Ruby circa 2017?

  # In Ruby 2.0 I can do Options 1 or 2:

  # Option 1 - Original Flavor, Most Explicit
  def self.first_message_in(message)
    implementation_here
  end
  private_class_method :first_message_in

  # Option 2 - Same thing but with class <<self instead of def self.
  class <<self
    private
    def first_message_in(message)
      implementation_here
    end
  end

  # Option 3 - In Ruby 2.1, "def" returns the method name as a symbol, so we can
  # actually embed the private_class_method call directly. Feels a bit like Java
  # or C#, but it is not without its charm.
  private_class_method def self.first_message_in(message)
    implementation_here
  end

  # Option 4 - Same as 3 but with private_class_method on its own line. This
  # SORTA makes it look we're like saying "private" for the following method,
  # which kinda feels nice, but also kinda feels misleading because it is not
  # saying "private" for ALL the following methods.
  private_class_method
  def self.first_message_in(message)
    implementation_here
  end

  # Continuation of option 4... If you didn't recognize private_class_method,
  # would you be misled into thinking this method was private, too?
  def self.other_method(message)
    implementation_here
  end
end