Cryo-CMOS circuit performance at 4K including both BEOL and FEOL characteristics has been investigated in a 65nm bulk CMOS for the first time. ON-current (Ion) of n/pMOSFET are improved +25%/+9% with excellent gate modulation (Ion/Ioff=∼109). Cu line/via resistances decrease with temperature due to reduction of phonon scattering, and -75%/-20% lower resistances are obtained at 4K. It is revealed that there is no inter-line capacitance change and no severe Joule heating effect (JHE) of Cu BEOL at 4K. The newly developed 4K-SPICE model including BEOL characteristics enables accurate CMOS circuit design at 4K, giving 5 ∼ 40% faster operation of RC line with clear dependence on driver-size and interconnect-load.