Believe it or not, I hadn't seen Star Treks 2 and 3, and had seen only a few segments of 4. That got remedied over the last few days.
Star Trek II: The Wrath of Khan in 25 words or less:
A simple story, perhaps, but one with great emotional resonance and greater depth than expected, in both character and narrative. B+.
Star Trek III: The Search for Spock in 25 words or less:
This story deserved a better movie. Something about the tone clashed with the plot, and it squandered its opportunity for deep emotional impact. B-.
Star Trek IV: The Voyage Home in 25 words or less:
It's not a great movie, perhaps, but it's the movie it wants to be, demonstrating that lower ambitions may result in a more satisfying product. B.