I was wondering if you waited a very long time, would the calendar eventually roll over to the 4th era?

I think that someone tried that using the console and the answer is no, the calendar does not roll over to the 4th era.

Keep in mind that each era is a different length of time, depending on when the next line ascends the throne.
Who can say what the future holds for the realm, what with the Marooned Dragon doing his best to ruin the landscape of man. We might already be in the 6th era, they just havent told us yet.

No.. thats not possible, because u don't know how many years each Era have.

-The First Era had 2920 years
-The Second Era had 897 years
-The Third Era had at the moment 433 years, so the Third Era can have 3000 years, so u don't know the exact number of years each era can have.

The above is not correct. The Third Era ended with the death of the Septim bloodline, and the Fourth Era begins at the end of the Oblivion Main Quest. See the http://www.uesp.net/wiki/Lore:Fourth_Era for confirmation of this.

you're right... my mistake. but thanks for the advise