Component of the SMC5-SMC6 complex, that promotes sister chromatid alignment after DNA damage and facilitates double-stranded DNA breaks (DSBs) repair via homologous recombination between sister chromatids. Interacts with SMC5, SMC6A or SMC6B. The SMC5-SMC6 complex is composed of the SMC5 and SMC6 heterodimer attached via their hinge domain and from the non-SMC subunit NSE4A or NSE4B (By similarity). Not expressed in seedlings, rosette leaves and floral buds. Belongs to the NSE4 family.