Despite the near-certainty of cancellation, ratings in the cellar and nothing left to lose, the fourth and final season of Star Trek: Enterprise was unanimously hailed as the best. After ending season 3 with a mind-boggling cliffhanger, series creators Rick Berman and Brannon Braga handed show-runner duties to executive producer Manny Coto, who rejuvenated the flagging franchise by bridging the gap between Enterprise and the future developments of Star Trek: The Original Series. By recruiting lifelong Trek experts Mike Sussman and the husband-and-wife team of Judith and Garfield Reeves-Stevens to his writing staff, Coto ensured that political events in the Enterprise timeline would lead to a "coalition of planets," thus forming the Federation cornerstone of Star Trek's future. But first, Coto had to find a way to extract Capt. Archer (Scott Bakula) and his battle-worn crew from an alternate timeline--the result of the continuing Temporal Cold War--in which the Nazis have invaded U.S. soil in 1944. In the normal Enterprise timeline, political upheavals have left relations between Vulcans, Andorians, Tellarites, and Humans in a state of near-disastrous chaos.

Into this blazing cauldron of action-adventure, Coto and staff introduced story arcs that connected to Star Trek's future, including a three-episode arc ("Borderland," "Cold Station 12," and "The Augments") in which Dr. Arik Soong (played by Next Generation alumnus Brent Spiner) and his superhuman "Augments" chart a tragic course that would lead, in future generations, to the creation of Spiner's cybernetic NextGen character, Data. "The Forge," "Awakening," and "Kir'Shara" returned T'Pol (Jolene Blalock) to her native Vulcan, where encounters with the legendary philosopher Surak, and zealous sect called the Syrannites, lead to pivotal history with the Vulcan High Command. In subsequent episodes, Phlox (John Billingsley) would discover the reason why some Klingons lack "cranial ridges" (thus solving a vexing Star Trek mystery), and "In a Mirror, Darkly" marked and eventful return to the "Mirror Universe" from the original series episode "Mirror, Mirror," for what Coto aptly describes (in the bonus featurette) as a two-part "romp," complete with a "Mirror Universe" title sequence, the reappearance of the U.S.S. Defiant from the original series episode "The Tholian Web," and a glorious recreation of a "Constitution Class" starship bridge that gave long-time Trekkies a breathtaking rush of nostalgia.

In the closing episodes, the formation of the Federation is threatened by a radical xenophobe (Peter Weller) whose isolationist tactics lead Trip (Connor Trinneer) and T'Pol to a future of interspecies parenthood, and while the series-ending "These Are The Voyages..." is considered a disappointment by some, it provided a suitable Next Generation tribute to Star Trek's past, present, and future. Considering the daunting challenge of tying up loose ends while looking forward in a way that demanding fans could appreciate, it's fair to say that Enterprise reached a satisfying conclusion that its cast and crew can be proud of.

The True Prequel Season

I've read much well-reasoned criticism of the fourth season, covering the whole spectrum from praise to revilement, and as a loyal Trekker for 30 years I feel compelled to offer my opinion as well.
Liking or disliking this season (and the whole series for that matter) really depends on what you're watching it for. If you want a true prequel to ST:TOS, this is definitely the season to watch. The stories provide much insight and historical background for the shift of Vulcan attitudes from arrogant to pacifist; the continuing consequences of the Eugenics Wars (along with the original theoretical conception that would culminate in the creation of Data); an explanation for the smooth-headed Klingons in the ST:TOS (and it was actually reasonably plausible!); an explanation of what happened to the Defiant; and most importantly, the genesis of the alliance between Earth and its nearest neighbors and ultimately the United Federation of Planets. In sum, this season, more than the rest, really does fulfill the raison d'etre for the prequel series like no other, and on those terms anyone familiar with the Star Trek franchise will find it most satisfying.
If, on the other hand, you are looking for character development, you are better off watching the third season. Some character aspects, e.g. flaws, only come out when the person is under extreme stress, and the third season was the ultimate stress test for the Enterprise crew. With one exception (the denouement of the relationship between T'Pol and Trip), the real development in the fourth season is in the history of the Federation, its enemies, and its allies, rather than of the individual characters.
No season of any of the Star Trek series' can go without some criticism, and there are some disappointing aspects. First, the storyline of the season opener (and the closer of the third season) seemed rather anticlimatic and the whole alien-Nazi theme marred what should have been Enterprise's finest hour--the triumphant arrival after saving Earth from destruction in a mission more difficult and dangerous than any other in the 40 years of the Star Trek. Second, the relationship between T'Pol and Trip fizzled out without any good reason; first Trip lets her go, then he wants her back, then T'Pol is available again and wants him back but he's not interested. Huh? A grand opportunity for exploring relationships between humans and Vulcans on a far more intimate scale was lost. Third, the final episode. The original concept of the show within the show had merit, but that was an experiment that should have been tried in the middle of series, not the finale, and definitely not what should have been the most important episode in all of Star Trek. The founding of the UFP is just too important not to take place in real time; Trip is killed off for no good reason; the reprise of Riker and Troi just didn't work (and since neither Frakes nor Sirtis have aged well, neither were especially pleasing to look at); and the final voyage of Enterprise was a non sequitur to the personal crisis Riker was undergoing (see The Pegasus, ST:TNG). After such a fine season, it is regrettable that seemingly everything about the finale was wrong. I suppose I am fantasizing at this point, but I would like to see the finale recalled entirely and a new, two-hour finale (movie?) be produced in its place.
Should you, the reader of this review, buy this season and watch it? If you are a Star Trek fan, absolutely. But to really understand and appreciate it you need to have watched the 1st and 3rd seasons. It would also be a really good idea to have watched The Original Series, especially if you want to understand the "Mirror" arc. Happy viewing.

They Got It Right!

It is a shame that Star Trek: Enterprise was not appreciated for the fine series that it was. The fourth season, after a tense and creative third season, was superb; the writers got it right and gave Star Trek fans what they wanted: substantial, intriguing stories involving Vulcan society, the mystery of the Klingons' appearance, the origins of the alliance that become the United Federation of Planets, and more. Let's be fair, though, about the first and second seasons...they weren't THAT bad, either; I found that many of the episodes were well-written, well acted, and quite entertaining. (Noteworthy were "Fusion," "Carbon Creek," and "Stigma"). It's very unfortunate that there will be no more "Star Trek: Enterprise" episodes to carry us through the Romulan Wars, the UFP's founding, the changes in Vulcan life, and all the possibilities that the writers were just starting to explore. It is a great loss.

Entertaining, but ultimately somewhat unsatisfying

SPOILER ALERT!

I watched all four seasons in a marathon run, over the course of about two weeks; and while much of it was entertaining, it left me a bit cold. Particularly the series finale and over-arching segue into the Star Treks "to follow" (as everyone knows, this series chronicles the years leading up to the original Star Trek series).

The fourth season acts as a primer for what is to come, by providing the back story for many of the episodes in the original series. In so doing, it sort of failed the series Enterprise, by divesting itself of the character arcs that it had built over the previous three seasons, and focusing on the "future" of the franchise.

As for the finale, one reviewer here hit on a real problem: Commander Tucker's death was terribly contrived. While it is commendable that the series producers were willing to kill off one of the series' core characters, the manner in which they did it was a stretch. Tucker effectively commits suicide when, after a minor and easily resolved skirmish with an intergalactic group of petty thugs, he kills the group and himself to "protect" the captain. Considering the number of times the crew encountered far more serious threats to themselves and the ship (and humanity itself!) and managed to extricate themselves without resorting to such drastic measures, his actions were illogical in the extreme. Furthermore, the abrupt insertion of Riker and the TNG crew into the finale rankled me, as it did many viewers. It just seemed to cheapen the series (Enterprise, that is) by having it told to us through the eyes of Riker and Troi.

Nevertheless, the last season of the Star Trek franchise (for now, one must assume) did have its share of good story lines and resolution. No selfr-respecting fan of the series can justify not watching it, and, as such, it is recommended.