which model do you have exactly ( "kindle fire" usually refer to the two devices launched in 2012
after that they begna calling them just " fire" )
the 2012 models cannot / will not uppdate to OS 5.x AFAIK
if you can post the exact model number ? identifier - which may be on the "about" page in settings, that will help
or look here:
https://www.amazon.com/gp/help/custo...deId=201263780
nb as per that page there is no such thing as a kindle fire 4th gen ?