Seven of Nine is a Borg who is freed from the collective and quickly becomes one of the most interesting characters on the ship. She brings into question what it means to be human and how to discover your humanity after being raised without it. The episodes that focus on her are some of the best of the season.
There's also a lot of development between Tom and B'elanna. It's the first real romantic relationship on the ship that we see consistently being developed. This is the season where they finally come together and have an official relationship. It's a nice common thread that keeps moving forward throughout most of the episodes in Season 4.
Not to mention that the Doctor once again gets a couple cool episodes with things that only his holographic self can do. He's still my favorite character.
One of the most exciting developments of the season is that Voyager finally communicates with the Alpha Quadrant through an alien communications relay before once again getting cut off. But they got updates from home, and now Star Fleet knows they're alive and on their way back.
It's nice to see big progress on the trip back to the Alpha Quadrant. I'm still having a good time watching the show and I'm excited to start season 5! I'm glad this Star Trek clicked with me. It's making me really excited for the new Star Trek that CBS is launching later this year.