I've enjoyed this show since the beginning, and considering it's a kids show, I've always found it to have some surprising maturity and depth. And the things that really annoyed me about some of the early Dr. Who episodes (e.g. fart jokes and childish "gross-out" moments) were always present but less annoying in the Sarah Jane Adventures, because of the context and target audience. Well, I'm happy to say that season 4 not only does away with childish gross-out-ness (for the most part), the stories have a more serious and mature tone than seasons past, with more emphasis on character (although they have always been very good in this respect). Also some noticeably improved acting from Tommy Knight, and very solid performances from the other two. Even a moment from Rani that moved me to tears. Clyde, as always, is my favorite, and he has a great season here. And Elisabeth Sladen, of course, is simply stunning as the Sarah Jane we've known and loved for so many years. She will be very missed.
There was only one episode I didn't care for (and it was a huge disappointment, being the one I was looking forward to most), but it still had some fine moments. All five of the other episodes were truly stellar - much better than I was expecting even after three very good seasons. In fact, I honestly think with this season, the Sarah Jane Adventures has finally arrived - yes, it is AS GOOD as Dr. Who, and considerably better than some of it.
Even if you haven't watched previous seasons, do yourself a favor and grab season 4. Dr. Who fans won't be disappointed.
And yes, there is a season 5, currently airing in the UK. Obviously there will be no more, so I hope it lives up to the very high standard set in this season.