Yes, the gap between Season 4 and Season 5 is 18 months, so the 18 months could be a reference to the shooting-down of Air Force One in Season 4.

But how do Graham and his pals fit into the Season 4 story line?

My thought was simply that the 18 months referred to the amount of time that their puppet Logan had been in the White House. But I agree that Czar's wording seems to imply that the wise men played a role in Season 4's events.

Overall timeline:

Season 1 to Season 2 is 18 months; the opening slide of Season 2 says "Eighteen months later".
Season 2 to Season 3 would have to be about 3 years, since Palmer was in the middle of re-election debates (October?), and he was just running for the nomination in Season 1 (set on the date of the California presidential primary, which was held in March at that time).
Thus, Season 1 to Season 3 would be about 4.5 years.
Season 3 to Season 4 was about 18 months, according to the prequel.
Season 4 to Season 5 was 18 months, as noted above.

Thus, Season 1 to Season 5 is about 7.5 years.