Simplicity
S4 is just a practical superstructure built on a solid IPA foundation. In accordance with the Occam’s razor principle, it is designed to be the simplest possible way of writing the sounds of English with symbols.
This has meant leaving a lot out.
It only covers the way English is pronounced in middle-class circles in America and England. It does not cover the way it is pronounced in Ireland, New Zealand etc., or the way my Great Aunt Madge who lived in Worthing used to speak.
If Cockney were to be covered, a Dark W would need to be added, as in li·‛w.
In this presentation, the use of metalanguage has been kept to a minimum. For instance, it will not help an EFL teacher to know that a Dark L is also called a “velarized alveolar lateral approximant”.
To quote Voltaire “mieux est l’ennemi du bien”.