In this fourth movie in the Thin Man series, the familiar formula still works pretty well, making "Shadow of the Thin Man" an enjoyable feature with plenty of wit, an interesting mystery, and most of all Nick and Nora. It's hard to think of any other screen couple that worked together better than William Powell and Myrna Loy. All it takes is a few seconds of seeing them interact before you feel as if you are in the company of old friends.
The story and setting make use of Nick's fondness for the horse races, and this also allows for an entertaining assortment of characters. The mystery has several twists and turns, and the story developments alternate with lighter stretches of Nick and Nora being themselves. Besides the race track, there are some other imaginative settings that help in creating an atmosphere that is both believable and interesting.
Most of the other characters are pretty straightforward, but Sam Levene gets quite a few good moments as the police lieutenant. Barry Nelson also has a decent role as a reporter. A very young Donna Reed gets a fair amount of screen time, but her character is not as interesting as the others. It's also interesting to see Stella Adler in one of her rare screen roles.
This one is a cut below the earlier movies in the series, but it's still good fun. As well as the familiar combination works, there weren't a lot of reasons to make significant changes.