BOUND
Orion slave girls enslave the Enterprise crew by using their pheremones. Cue whacky shennanigans.
That's it. That's the plot. And thanks, Star Trek, but I hate it.
MIRROR UNIVERSE ARC
Cheese. Delicious, delightful cheese. A duology set in the Mirror Universe as we see evil versions of the cast do evil things to evil people, or at times, innocent ones, who get to die in evil ways. Get everything from evil!Archer losing his sanity, to evilsexy!Hoshi doing evil things, and everyone hamming it up. It's cheese, and I love it.
Also we get to see tholians and an updated gorn, so that's nice.
TERRA PRIME ARC
Mostly positive, but this really feels like it should be a three-parter. Because it's a payoff for the season and arguably the series as a whole, but it doesn't feel like it gets the airtime it deserves.
Anyway, the Federation's precursor (forget its name - it's like a proto-Federation) is about to be formed, but anti-alien terrorists seize control of an array on Mars and demand all aliens leave Sol. It's payoff for the xindi in season 3, and the resulting anti-alien sentiment seen at the start of season 4. It draws attention to how it's mankind at its worst and best, hence one of the episodes being named "Demons." There's some really powerful stuff here, such as Archer's speech at the end, and T'Pol and Trip losing their child (okay, technically not theirs, but a hybrid formed from their DNA, and one they mourn).
On the other, something feels lacking. Again, like it's not fleshed out enough. Again, this is meant to be THE moment that humanity can show its better self to the galaxy, but it's just a bunch of twats being terrorists. But overall, it's mostly solid, if partly due to extremely powerful key moments.
THESE ARE THE VOYAGES
This has been cited as one of the most hated episodes in all of Star Trek. Having seen it, I disagree (it's no Shades of Grey for instance), but it's still heavilly flawed, and as a series finale, you could do MUCH better.
I think the setup is good, actually - Riker watching a holodeck recording of the NX-01's final voyage, and Archer's speech at the declaration of the Federation (not technically the Federation at this point, but in function, if not name). That's a great setup by itself, as it forwards the plot of Enterprise, and shows its weight in the larger Star Trek canon. However, what hinders this is the fluff - in TNG, it's set during the Pegasus episode (which I haven't seen), as Riker is watching events in the past to guide his actions in the present for vaguely explained reasons). Simultaniously, in the past, the crew has to rescue Shran's daughters from space pirates. Both of these elements feel superfluous to the larger plot.
And then there's Tripp's death. Tripp, who kills those pirates by comitting suicide, getting a bit of mourning, and, yeah. Tripp dying in of itself isn't a bad thing, but the way it's done here...it's not one of the worst character deaths in fiction, but it's still pretty lacklustre. I think T'Pol mourned more for her 'daughter' in Terra Prime than she does for Tripp here. And that there's a six year time jump from TERRA PRIME and the two just drifted apart off-screen doesn't help much, when literally the last shot of TERRA PRIME is them mourning their deceased 'daughter' together.
Still, it's got the 'feels' to see the Federation finally established, and the closing shot of the TNG, TOS, and ENTE Entrprises flying through space as the "final frontier" monologue is shared between Picard, Kirk, and Archer...it's beautiful. However, the episode could have been more. I get the sense that this was always planned, and with Enterprise cancelled in season 4, they simply cut to the ending. I don't think that's a bad idea per se, but it could have been done better if the 'fluff' was cut.
So, yeah. Overall, enjoyed season 4. Not as good as season 3, but much better than 1 and 2. It's actually elevated Enterprise as a whole for me, so if I was ranking Star Trek shows, it would go TOS>Enterprise>TNG>Discovery at this point. Granted, Enterprie is now the only Star Trek show I've seen all of, so that could change. But overall? Pretty good. At least if you can slog through the first two seasons.