This paper uses a regression discontinuity model to examine the effects on kindergarten entrance assessments of the Kalamazoo County Ready 4s (KC Ready 4s) program, a half-day pre-K program for four-year-olds in Kalamazoo County, Michigan. The results are based on test scores and other characteristics of up to 220 children participating in KC Ready 4s, with data coming from both 2011–2012 and 2012–2013 participants in the program. The estimates find consistently statistically significant effects of this pre-K program on improving entering kindergartners’ math test scores. Some estimates also suggest marginally statistically significant effects of KC Ready 4s on vocabulary test scores. No statistically significant effects are found on letter-word identification test scores, due in part to the small available sample size, but some of the point estimates are large. The program does not appear to have large or statistically significant effects in improving children’s behavioral assessments. The overall average effects of KC Ready 4s on the three academic test scores are large, at an effect size of at least 0.52. This is toward the high end of effects found in previous studies of short-term effects of pre-K programs. These estimates also are consistent with program benefits exceeding program costs.