This has me stumped. Even the Great Google can’t help, maybe I’m not asking it right…

It’s a letter sequence that starts with four letters that has to be continued with the next four.

EG: T F F S . . . . ? answer being S E N T
Three Four Five Six – Seven Eight Nine Ten
J D N O. . . . ? S A J J Months of the year in reverse

I am stuck on H R T E . . . . ?

Final question in a quiz I can’t complete! Possibly something so obvious I’m about to be very embarrassed.

US Presidents?
Hoover, Roosevelt, Truman, Eisenhower.
So next 4 is Kennedy, Johnson, Nixon, Ford.

Gah! And I went through Prime Ministers to no avail!