Why does ETOPS not apply to older 4 engine aircraft?
I think it's because the last 747-400 was built in 2009.


  §121.162   ETOPS Type Design Approval Basis.
  
  Except for a passenger-carrying airplane with more than two engines manufactured prior to February 17, 2015 and except for a two-engine
  airplane that, when used in ETOPS, is only used for ETOPS of 75
  minutes or less, no certificate holder may conduct ETOPS unless the
  airplane has been type design approved for ETOPS and each airplane
  used in ETOPS complies with its CMP document as follows:
  
  (a) For a two-engine airplane, that is of the same model
  airplane-engine combination that received FAA approval for ETOPS up to
  180 minutes prior to February 15, 2007, the CMP document for that
  model airplane-engine combination in effect on February 14, 2007.
  
  (b) For a two-engine airplane, that is not of the same model
  airplane-engine combination that received FAA approval for ETOPS up to
  180 minutes before February 15, 2007, the CMP document for that new
  model airplane-engine combination issued in accordance with
  §25.3(b)(1) of this chapter.
  
  (c) For a two-engine airplane approved for ETOPS beyond 180 minutes,
  the CMP document for that model airplane-engine combination issued in
  accordance with §25.3(b)(2) of this chapter.
  
  (d) For an airplane with more than 2 engines manufactured on or
  after February 17, 2015, the CMP document for that model
  airplane-engine combination issued in accordance with §25.3(c) of this
  chapter.
  
  §25.3   Special provisions for ETOPS type design approvals.
  
  (a) Applicability. This section applies to an applicant for ETOPS type
  design approval of an airplane:
  
  (1) That has an existing type certificate on February 15, 2007; or
  
  (2) For which an application for an original type certificate was
  submitted before February 15, 2007.
  
  (b) Airplanes with two engines. (1) For ETOPS type design approval of
  an airplane up to and including 180 minutes, an applicant must comply
  with §25.1535, except that it need not comply with the following
  provisions of Appendix K, K25.1.4, of this part:
  
  (i) K25.1.4(a), fuel system pressure and flow requirements;
  
  (ii) K25.1.4(a)(3), low fuel alerting; and
  
  (iii) K25.1.4(c), engine oil tank design.
  
  (2) For ETOPS type design approval of an airplane beyond 180 minutes
  an applicant must comply with §25.1535.
  
  (c) Airplanes with more than two engines. An applicant for ETOPS
  type design approval must comply with §25.1535 for an airplane
  manufactured on or after February 17, 2015, except that, for an
  airplane configured for a three person flight crew, the applicant need
  not comply with Appendix K, K25.1.4(a)(3), of this part, low fuel
  alerting.