I know several people who have broken 4 hours on a century ... there's a group of racers in Manitoba who do it just about every year on the Muddy Water 100 in August. But then ... they ride in a large paceline on Manitoba terrain (the flattest land I've ever laid eyes on!).
There's a big century ride in Belgium, Holland, or northern France (not exactly sure where) where they set the pros (or ex-pros) out first and then regular cyclists can ride and can see if they can keep up to the pros. A friend of mine rode that and did it in under 4 hours ... but again ... big pacelines and that area is quite flat ... not as flat as Manitoba, but pretty flat.
Also, were they recording TOTAL TIME (which is the correct way of calculating century time) or ride time (which is the wrong way to calculate century time)? We've got to record TOTAL TIME because some riders can do a century non-stop and some riders need breaks to complete the century. If we need breaks (I do), we've got to count them.
I will add this ... many Roadbikeriders are also randonneurs or 24-hour racers, etc.. So they're quite used to riding long distances, and a lot of them are very good at it.