The highlight of The Sarah Jane Adventures series 4 was undoubtedly the story Death of the Doctor. It featured the eleventh Doctor, played by Matt Smith of course, a darker side of UNIT, and former companion Jo Grant.
She left the Tardis because she was getting married, so she is naturally astounded that the Doctor allows married companions on the Tardis now. It's clear this story takes place after the events in the Doctor Who special, A Christmas Carol, since Amy and Rory are still on their honeymoon.
The other five stories are excellent as well, although there are times when they deliver the life lesson with a bludgeon. Thankfully this problem is specific to certain writers and isn't indicative of the overall production.
The younger characters, Clyde and Rani, were given much more story time in series 4 than in previous ones, which allowed for more character development. The super genius, Luke, has gone away to university which gave Clyde and Rani a chance to shine. They actually had one story, The Empty Planet, all to themselves, literally. In a couple of stories, the young assistants were the ones rescuing Sarah Jane, which was a nice turn about. I hope the trend continues in series 5.