The title says it all-- four sisters (and their widowed dad) spend the summer in a vacation house on the property of a rich woman and her son, Jeffrey. The girls soon befriend Jeffrey, and help him with some fairly standadrd mg-book-problems with his family.
This book is written to be intentionally "old-fashioned," which I applaud, but not at the expense of good characters, which is what happens. The plot is fairly predictable, and the four sisters are fairly cliche, with the exception of maybe Syke, who comes accross as wooden. If you're looking for a good four-sisters-in-summer story, try The Exiles by Hilary McKay, which is a thousand times more entertaining.