This cross-sectional study investigates, from a processability theory (PT) perspective, a developmental sequence of acquisition of morphosyntactic constructions with advanced learners of Japanese as a second language (L2). Using Lexical Functional Grammar (LFG)-based analysis, this study hypothesizes formal descriptions of a construction that Kawaguchi (e.g., 2007) categorizes as belonging to Stage 5 in the processability hierarchy (PH)—the ‘wa’ and ‘ga’ particle distinction in matrix and subordinate clauses. The hypothesized descriptions in the present study suggest that the construction actually belongs to Stage 4. L2 learners were tested to determine whether they sequentially followed the newly hypothesized PH, as PT predicts. Although the results support the newly hypothesized developmental stages, distributional analysis indicates the existence of intra-stages within each stage in PH based on the complexity of form-function mapping, as Mansouri (e.g., 2005) suggests. This study also reveals that Stage 5 still needs to be refined through further LFG-based examination.