Purpose: This study evaluated the effects of an automated, small-group intervention designed to teach preschoolers challenging vocabulary words. Previous studies have provided evidence of efficacy. In this study, we evaluated the effects of the program after doubling the number of words taught from 2 to 4 words per book. Method: Seventeen preschool children listened to 1 prerecorded book per week for 9 weeks. Each storybook had embedded, interactive lessons for 4 target vocabulary words. Each lesson provided repeated exposures to words and their definitions, child-friendly contexts, and multiple opportunities for children to respond verbally to instructional prompts. Participants were asked to define the weekly targeted vocabulary before and after intervention. A repeated acquisition single-case design was used to examine the effects of the books and embedded lessons on learning of target vocabulary words. Results: Treatment effects were observed for all children across many of the books. Learning of at least 2 points (i.e., 1 word) was replicated for 74.5% of 149 books tested across the 17 participants. On average, children learned to define 47% of the target vocabulary words (17 out of 36). Conclusions: Results support including 4 challenging words per book, as children learned substantially more words when 4 words were taught, in comparison to previous studies. Within an iterative development process, results of the current study take us 1 step closer to creating an optimal vocabulary intervention that supports the language development of at-risk children.