I am pretty sure that the Comet-4 did see Transatlantic service with BEA/BOAC. Mexicana also had a few Comet-4s that must have flown t/a from Mexico to Europe.
The Comet-4 however found its niche with Dan-Air as a second tier operator well into the late 70s, I believe they snapped up most of the airworthy Comet-4s as they became available. Other Comet-4s were operated by MEA in the Middle East and by East-African.
The Comet is one of my favorite a/c, beautiful graceful lines if not a bit cramped inside compared to its competitors.