We design a multi-connectivity scheme based on the cooperation between the networks of the fourth generation (4G) and the fifth generation (5G) of mobile technologies, to improve the reliability of Ultra-Reliable Low-Latency Communication (URLLC) services. While 5G system is characterized by a short Transmission Time Interval (TTI) and fast retransmissions within the delay budget, 4G system has a large TTI so that only blind retransmissions are possible. We develop an optimization problem that couples this multi-connectivity with an adapted number of replications, scheduled by exploiting Power Domain Non-Orthogonal Multiple Access. The resulting optimal scheme is evaluated in an outdoor urban macro scenario for different reliability targets and user locations in the cell. Results show that the scheme minimizes the number of replicas while achieving the performance targets for both URLLC and eMBB users.