Four-day school weeks are becoming increasingly common in the U.S., but their effect on achievement is not well-understood. Using a difference-in-differences approach, we conduct the most representative student-level analysis to date of the effects of four-day weeks on student achievement and within-year growth using NWEA MAP Growth data. We estimate significant negative effects of the schedule on spring reading achievement (-0.07 SD) and fall-to-spring gains in math (-0.05 SD) and reading (-0.06 SD). The negative effects of the schedule are larger in non-rural schools and for female students, and they may grow over time. Policymakers and practitioners will need to weigh the schedule's varied effects across contexts in their decisions regarding how and if to implement the policy.