The Comet 4s never were grounded (as in the term usually used when prevented from flying due to a safety concern). That was earlier versions - the 4 was the result of all the changes after the fatal structural failures of the previous marks.

Comet 4s continued to fly well into the 80s with Dan Air and my first ever flight was on one on a school trip to Rome in the mid-70s. Memorable for one of the young teachers getting pissed on the outbound flight and she had to be helped off the plane by a number of us pupils. Imagine the Daily Mail headlines if that happened on a school trip today!!