I think the theme in Season 4 was similar to the theme in Season 5. Whereas 5 was growing up and realizing the things that you love the most, 4 was more of gaining independence, strained relationships, inferiority complexes and the overall feeling of change and despair.
For Buffy, it was moving out on her own and having to leave behind the mother and father roles that Joyce and Giles played in Seasons 1, 2 and 3. Even though they were there to guide her, Buffy needed this to be a stronger person, not to have to confide in someone to make every decision. With the appearance of Sunday and Adam, Buffy soon realized that maybe she wasn't the best or that she needed to try a lot harder to fulfill her destiny. The whole Initiative thing could be seen to Buffy as someone trying to get 'all up on her' territory. Angel had left and I believe that Buffy needed someone there to love her, thus Riley. Soon Buffy cut off her friends and spent most of her time with Riley, which distanced herself from the ones she loved most. She learned to accept things and in the end she pulled it together and regained the trust and admiration of the Scoobies. (Even though I think that the Scoobies have hindered her ever since her move to Sunnydale. After all these years, you see that Buffy doesn't fully trust herself and she has to rely on her friends more and more.)
Willow, the Ivy-League accepted who went to UC-Sunnydale experienced the leaving of Oz, a growing intimacy with Tara and more power. Willow had to deal with many changes but she got her stuff together and she wasn't afraid of the person that she truly was. I think that over the years she's grown tremendously but still has the shy and sweet characteristics that we've loved since Season 1. For some time Willow felt inferior to Veruka(sp?) because of the attention she received from Oz and somewhat from Buffy when she outshined Willow academically in Maggie Walsh's Psych 101 course at UC-Sunnydale. (I think Season 4 and 5 Willow is the Willow we should have had in Season 7, the strong and confident Willow)
Xander had to live in his parents basement and pay rent, and got plenty of jobs which I believe taught him a great deal of responsibility and work ethic (In Season 7 he's been through promotions and earning a good living for a 22 year old.). But not going to college didn't give him a big self-esteem boost and he was probably, if not, envious of his best friends. His coupling with Anya I think was a huge turning point and gave him the companionship and love that he needed. I think Xander somewhat lacked the love that he needed from his family, but he's always received this from the Scoobies, who they consider to be a family (which is sweet) but he was cut off from this as were all the Scoobies for a great deal in Season 4. We also see an inferiority complex with him while he's working at the bar in Beer Bad and some of the more affluent and intellectual college-going try to show off and make him feel lower. But Season 4 Xander learned that just going to school doesn't make someone better and that's true.
Giles, the unemployed Hugh Heffner-like appearance he made in The Freshman showed the he was really lacking something without his job as both Librarian and Watcher and his relationship with Buffy. Giles felt useless as Buffy was growing stronger and she was becoming more of a leader. But I think Season 4 Giles transformed from stuffy British teacher-like character to more of a friend-somewhat stuffy British older guy who we hang out with but not in a creepy Lifetime movie way!
And in the end, they became even closer and all merged into a SuperBuffy (that's really sweet too!)! Then they watched a movie, fell asleep, had their dreams paraded by the First Slayer who tried to kill them but they became even closer and learned a lot about themselves in the process!
