It appeared that the writing staff had set up all sorts of things in Season 3 that would be followed up, presumably at length, in Season 4--Archer dealing with the psychological costs of the moral lines he was forced to cross in order to complete the Xindi mission, Trip and T'Pol's nascent relationship, Hoshi's torture by the Xindi-Reptilians, the consequences of T'Pol's trellium use. There could have been revisits of any number of dangling plot threads--the Illyrians, the North Star colonists, the unknown fate of the E2
Enterprise, etc.
But, in hindsight, I think Season 4 was hampered by the looming cancellation of the series, and the necessity of having to basically cram 4 years' worth of goodies into one truncated season. To make room for several episodes that Had To Be Made, a whole lot of ongoing plots with great potential were given short shrift, or just tossed.

Rich storylines like T'Pol's adjustment to post-trellium emotions, or Archer coming to terms with what he did during the war, were reduced to almost nothing. And the Trip/T'Pol relationship was all over the place--put on hold, ignored, turned into a soap opera, fast-tracked, given a tragic but uplifting ray of hope, then canceled.

My theory: You don't plan things like this. This is the kind of stuff that happens when your plans get derailed.
On the bright side, there was some great character development for Soval and Shran; I really enjoyed the Vulcan and Andorian arcs. There were unexpectedly interesting guest characters, like Dr. Antaak from the Klingon arc. We got to see Vaughn Armstrong as Forrest in "IaMD" even after Forrest was killed in the Vulcan arc--that guy was great no matter which universe he was in.

The Archer/T'Pol friendship was well-served. And there was "Observer Effect," which was my favorite.
Sure, there are storylines I wish I could have seen. I wish there had been seven seasons guaranteed, and the breathing room for the writers to explore plots and storylines without constantly worrying about ratings or cancellation.
Enterprise, of all the Trek shows, never had that luxury. Considering the pressure they were under, I think we got some real gems.
Just my .02