The -k4,4n -k5,5r first sorts numerically on the fourth field and then sorts on the fifth field in reverse order. The 5th field sort is a sub-sort that is it sorts the 5th field based on the sorted contents of the 4th field.The comma in -k separates the start and stop fields. In -k5r the start field is the 5th one and as no stop field is given it defaults to the end of the line. The sort manpage provides a better explanation.

No, I think you are wrong there. Your explanation is far clearer than man sort.

English is my first language, man help is my fifth !!

Many Thanks

Colin