4 hour century -- in the 1980's...

It has happened, I did a 4:15 at The Katy Flatland Century, maybe 1998 or so..the motorcycle leading the front group made a wrong turn and we had to loop back and work through the main field, might have been a 4 flat otherwise.

Contrast that to this past weekend when it took me 4 hours to do 56 miles...

I don't recall when cycle computers, as we know them, hit the market..some times and distances before then might be questionable

I have seen some impressive bike splits reading through the results of various events. I'm certain if any of these riders wanted to focus on that goal and picked the right event they could do it.
Katy was truly flat, the only "hill" of note was a highway overpass, also happened to be a calm day with comfortable temperatures..great day for a PB

It is definitely possible.  That is only 25 mph which is very good for us non-pros but you'll need to do at least that to race pro-1,2.

My Cateye Micro worked pretty well back then so it would be possibel to have measured accurately.  Except that one time it said my max speed was 88 mph.  Pretty sure that did not happen.

We have a ride here in Iowa called the Tailwind Century.  Bus takes you 100 miles into the wind and drops you off.  That could probably lead to some high speeds.