Season 4 is filled with double header and even triple header episodes and with 2 parallel universe episodes which are even more higher on the entertainment factor.
Season 4 starts off when the ever present Daniels pulls enterprise into earth's past to 1940 where the nazi's have taken a part of america and all of europe thanks to some time travelling aliens whos intent is to get home and dont care what means they do to archive it, enterprise must stop them and get shocked when they find Archer still alive.
pulled back into the present archer must deal with his troubles of recent and to his old flame who is just promoted to captain of columbia.
after that we find out more on the vulcans, their past and Administrator v'las and his ruthless nature which considered against typical vulcan behaviour is very unusual.
we also see the formation of the United Federation of Planets, the death of an important character while saving shran and his daughter.
A great series that was prematurely killed off. Shame about that.