Season 4 has split it's 22 episodes into 3 story arcs, with one key link. The season also ties in loosely with the release of Doctor Strange, adding mysticism to the MCU.
The first arc, featuring the Ghost Rider, was widely praised for bringing the show back on track after a fairly bleak end to season 3. Personally, I felt that these episodes relied too much on the Ghost Rider and his family and I didn't feel any particular attachment to them. Robbie Reyes (Ghost Rider) didn't feel like a good fit for the team, so it was obvious that he wouldn't be sticking around for the long haul.
In general, I still enjoyed the episodes, but I was a lot happier when that arc finished and the just completed LMD arc began.
The Life Model Decoy (LMD) arc was excellent. The events of the Ghost Rider arc allowed some big changes to occur, more or less unleashing the power of an ancient magical artifact into the world. This magic, combined with already established technology, leads to the creation of replica humans, indistinguishable from the real thing. It basically has a Battlestar Galactica feeling of people secretly being robots (and possibly not even knowing themselves), but doesn't get bogged down with the drama of "Who are the Cylons?"
The finale of this arc is one of the strongest episodes of the show to date and, to my mind, solidifies the show as the best current superhero TV show.