… I would guess that the next series would be at least 5 years to get a syndication package (or is the minimum 4 years?)
It's more to do with episodes than seasons. Rerun TV shows generally get stripped, that is they are shown Monday-through-Friday at the same time every day, usually in order.
Programming managers (the guys who decide what goes on the broadcast schedule) typically won’t air a series that runs in its entirety much more than about twice or two-and-a-half times in a year. At 2˝ airings per year, it lets the audience catch an episode in May that they may have missed in January, for instance. The extra half-airing might seem silly, but again, the business cares more about avoiding audience fatigue caused by timing rather than airing each individual episode equally.
Since there are roughly 260 weekdays in a year, a show with just about 100 episodes has the best chances of being syndicated. Too few or too many more episodes will keep a show from being repeated, without showing multiple episodes a day.
And airing multiple, hour-long episodes of the same show, five times a week is not the best way to reach one’s goal of keeping the audience from getting tired.