We investigate two-component ultracold fermions loaded into a decorated square lattice, which are described by the Hubbard model with repulsive interactions and nearest-neighbor hoppings. By combining the real-space dynamical mean-field theory with the numerical renormalization-group method, we discuss how a ferromagnetically ordered ground state in the weak-coupling regime, which originates from the existence of a dispersionless band, is adiabatically connected to a Heisenberg ferrimagnetic state in the strong-coupling limit. The effects of level splitting and hopping imbalance are also addressed.