Best idiom for private class methods nowadays, i.e in Ruby >= 2.1 and/or for values of "nowadays" significantly greater than 2014?

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 `def self.method` dropped in favor of `class
  # <<self; def method`. Note that "private" has additional semantics; all
  # methods defined after in in the block will be private, not just
  # first_message_in.
  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

  def self.other_method(message)
    # Like this, for example... other_method is public. Would you be misled into
    # thinking this method was private, too?
    other_implementation_here
  end
end

Just realized the misleading bit in option 4 might be countered with an unnecessary backslash, e.g.

private_class_method \
def self.first_message_in(message)
  implementation_here
end

It's syntactically unnecessary and a little bit distracting but I think maybe the distraction admirably serves a purpose: calling direct attention to the fact that "hey, this is ONLY attached to the next line, it does NOT change scope."