We study subsystem-structure selection in small interacting quantum systems. We compare two criteria: (i) maximization of cross-boundary correlations and (ii) minimization of dynamical coupling across subsystem boundaries. Using exact diagonalization of asymmetric spin chains, we show that these criteria can favor different tensor factorizations of Hilbert space. We derive the crossover parameter λ* at which the preferred factorization switches. The existence of this regime demonstrates that subsystem structure need not be uniquely determined by dynamical stability alone. This paper is part of the Relational Emergence Model (REM) series (REM4 of 5).