Gardner's aim in this slender volume is to introduce the student of philosophy not only to the Four Books themselves, but also to Zhu Xi's influential Neo-Confucian commentaries, through which Chinese students have traditionally approached and interpreted these classic texts. While Gardner does not translate Zhu Xi's commentary directly, his own commentary is peppered with quotations from Zhu Xi, and Gardner explicitly adopts a Neo-Confucian stance when explaining how the passages from the Four Books have traditionally been read. This choice does provide an effective introduction to the Neo-Confucian commentarial tradition, and to Neo-Confucianism generally, but at the expense of occasionally obscuring the original meaning of the texts.
By contrast, Edward Slingerland, in his translation of Confucius' Analects, makes use of a range of traditional commentaries, together with modern textual scholarship, in his own attempt to make sense of the Analects (in terms of their likely original meaning, to the extent to which this can be reconstructed). There are certainly advantages and disadvantages to both of these interpretive strategies, and there is likely room for both in preparing contemporary translations and editions of classic Chinese texts.
While Gardner's The Four Books largely accomplishes what it sets out to do, I would have preferred an edition which contains the complete versions of Great Learning, Analects, Mencius, and Doctrine of the Mean, even if this meant expanding the work to multiple volumes. Also, since Gardner in many cases merely paraphrases Zhu Xi's commentary, it would have been preferable to simply include a translation of Zhu Xi's commentary alongside the original text, supplemented where needed by Gardner's own notes or additions (perhaps noting those cases in which other commentators have disagreed greatly with Zhu Xi). Nevertheless, Gardner has produced an excellent introduction both to the Four Books themselves, and to the Neo-Confucian commentary tradition of Zhu Xi, which colored the reception of these texts in China for hundreds of years (and which still does).