dernieres decennies vs decennies dernieres

Carl, ‘ these last four decades ‘ follows the same pattern as ‘ these last few days,years ‘ in the linked lesson : 

            “ To express these last few ...In French, these last few days, years... is expressed as ces derniers jours, ces dernières années. 

 

Here, the meaning of the expression is "these latest days", hence dernier being placed before the noun. 

Note that we don't use quelques to express few. 

Beaucoup de choses ont changé ces dernières années. A lot of things changed these past few years.

J'ai beaucoup révisé ces derniers mois. I studied a lot these past few months.  “

 Dernier = final/previous (French Adjectives that change meaning according to position)

I asked my wife ( French ) for her immediate translation of ‘ these last four decades … ‘, the response  ‘  Ces quatre dernières décennies ‘.