The fourth season was, in a word, amazing. I had said before that I didn't like Donna in the Christmas special she appeared in, which is true. But let's just say she's much better in this season (thankfully). Indeed I quite liked her.
I won't do an episode by episode type of thing this time. That's because they are all good, with the one exception of "Midnight." That one was a dud. The library ones and the final three are the best, but they are all very good. There's one about Pompeii which makes me happy as a Classics major despite, well, being incorrect (or maybe not how could we know).
I loved how in "Turn Left" we get a glimpse of how things would have been were the Doctor not there to stop certain events happening, as Donna is pulled into an alternate world. And of course I was extremely happy for the return of Rose Tyler. It was foreshadowed throughout the season and of course I'm too curious for my own good and already knew about it thanks to the internet (though I didn't know any details... I'm not that bad).
The ending of the season was so great, but so bittersweet, I just have to talk about it. First of all we have all the companions that have appeared so far in the new series all converging in this one episode, which is brilliant. Donna gets a bunch of Timelord wisdom which I liked and creates a human/Timelord alternate Doctor as well. In the end the Doctor takes everyone home, and we wonder if Martha's going to join Torchwood (no clue... or even Mickey?). He has to erase Donna's mind so she doesn't go crazy with the Timelord brain and all... which is pretty sad because now she's back to being nothing significant. Worse still, she remembers nothing of her travels with the Doctor. That is so tragic!
My favorite thing that happens (and this is all out of order) is with Rose. So there's still the alternate Doctor and he's killed all the Daleks which the Doctor doesn't like so when he takes Rose and Jackie back to the parallel universe, he leaves the half-human Doctor there. Rose is of course not entirely pleased because, after all, she traveled to another universe to get back to the Doctor. While this half-human Doctor is pretty much the same, the Doctor will still be out there. I love the scene where Rose asks both of them to finish the sentence left hanging at the end of season 2 and the half-human Doctor finally says... whatever he says which prompts Rose to kiss him. So anyway... I imagine that Rose will not be fully pleased with the events at first. She'll think often of what the Doctor is doing and wish she knew. But eventually she'll realize here she has a man who is the Doctor (except that he will age and he's not the only one). He has the same thoughts and memories and looks the same and - most importantly - he is a Doctor who will not be afraid to love her. Sigh. Awwww. Now I know they'll have sex.
Footnote: I also just watched the two specials since then, "The Next Doctor" and "Planet of the Dead." The second one is definitely better, but I did like them both.