Yup, the fourth season of Enterprise was good, and a significant improvement in the first three (which had potential that was never lived up to)
But by then it was too late to regain audience share.
That fourth season was, I think, the inaugural season for the Washington Nationals. DC20, which had the rights to broadcast the games, kept shuffling the new eps of Enterprise off to the post-late night hours. I think I missed more than 3/4ths of the eps on first run because of that. When I was able to pick them up again I was well past the point of understanding how the individual eps fit into the overall show. The mirror universe eps with the Defiant, for instance. They were good, yes, but I had a hard time figuring out how they’d gotten there from a story arc perspective.
I had to buy the box sets to get all the eps watched in order and i was quite impressed by the several mini-arcs that made up the 4th season!
I liked the mini-arc format, from the arik soong arc, the mirror arc and the vulcan reformation arcs.