I think you're confused - perhaps you're remembering the season four two-parter "In a Mirror Darkly, Parts I & II"? But that was just two episodes, Enterprise never had a whole season set in a mirror universe. (And rather than being cancelled due to the mirror episodes, it was announced that Enterprise got the axe before those episodes even aired!)

Personally, I think the whole "mirror universe where good guys are bad and bad guys are good" premise is such a hokey sci-fi cliché that I think the Shattered Glass idea was doomed from the outset. It was a nice way to get some weird repaints, but beyond that I'm not sure the idea has legs.

I do think it works best in live-action television, where it allows actors a bit of fun playing something a bit different for a change (possession / mind-swap stories do the same trick), but I wouldn't be too disappointed if Shattered Glass was consigned to history.

 

It's not just those two episode, the entire season was set in that universe. I was even looking at the DVD box set at Wal Mart not too long ago. The complete series has two cases in the box, one for the main universe, and one for the mirror universe season. All of season 4 was set in that universe.

Did you actually watch any of those episodes or did you just look at the titles and figured the two with Mirror in them were the only ones set in that universe? They don't need to put Mirror in every title, they only did that for the fisrt two episodes but that doesn't mean it goes back to the main universe after that. MU Trip doesn't have a scar on his face and Tupal isn't a slave.

 

A complete list of all the mirror-universe Trek episodes can be found here:

Mirror Universe - Wikipedia

Only two Enterprise episodes are listed.

If you can back up your claims with supporting information - maybe you can list some titles of other episodes you think are set in the MU but are missing from that Wikipedia list - then I will happily stand corrected. It has been a while since I saw season four so I don't deny that I might be wrong on this. However a quick bit of googling seems to concur with my side, as well as all the other online guides (Wikipedia, StarTrek.com, Memory Alpha, Jammer's Reviews etc.).

I even found an online interview with the Enterprise producers stating that - because they enjoyed making the mirror two-parter so much - season five (had it been produced) would have had most of its episodes set in the MU.

 

Something else I forgot to mention before... they created a new opening title sequence specifically for season 4. It includes the sword threw the Earth symbol which is the emblem of the Mirror Universe Empire which replaces the Federation's symbol, the one which is based on the real world emblem used by the United Nations. Also entirely different music, different historical clips. It's still in the Enterprise style which goes totally against every other Star Trek series, but everything about it is new and that's a lot of work to put into something that's only going to be seen once.

I say once becauce the first episode didn't have an opening title, the First Contact scene played in it's place, the new title sequence was used from episode 2 onward.

Typically if if a show was going to create an alternate title sequence just for a single episode, it would be a warped version of the original with few changes actually done to it. But Star Trek made an entirely new one from scratch, the only similarity to the standard title being the use of real world history clips leading up to the Fictional future and the odd choice in music that doesn't sound like a Star Trek theme song. Though I'm not sure we can even call it real world history in the case of the Mirror Universe since apparently the first Enerterprise was a pirate ship. That's partly true, I mean the first ship in the real world with that name was a boat but not one that was used by pirates.

Anyway, shows do often make new titles like this when it's going to be used for at least a full season. So I'd suggest actually watching season 4, you'd know in the first two minutes if it was going to be mirror universe or main universe just by the opening credits sequence.

 

Enterprise Season 4 is not completely set in the Mirror Universe, only 2 episodes are, the ones mentioned earlier - yes they did create a new title sequence, which was a 'Mirror Universe' version of the regular Enterprise intro. Here is a link to Enterprise on the Netflix page, and here is a link to reviews of every episodes of Enterprise - just from those two links alone you'll find plenty of details of episodes, reviews and discussions which can help clarify the situation regarding Season 4.  

(Also I have watched Enterprise Season 4).