In any healthcare service, guidelines regarding the number of staff and how to respond to patient demand must be followed. In Chile, to ensure there is 24/7 care, coordinators use a manual allocation model system called “The Fourth Shift” (TFS) to assign staff. The model has a four-day shift pattern which allocates 48 h of work and 48 h of rest. However, scheduling healthcare workers is always a challenge, as there are administrative, legal and individual constraints. A balanced shift assignment, meaning one that considers work hours and specific staff requests, has a significant impact on an overall work environment. To find a fair balance, this paper proposes a two-phase heuristic. The first is a constructive phase and the second is a local search phase. This paper simultaneously incorporates six Key Performance Indicators (KPIs) and chance events aiming at leveling the workload for healthcare workers. The heuristics are validated with one-month shifts for a healthcare service with 12 nurses. The results validate the effectiveness of the proposed approach by disrupting the solution with five cumulative scenarios.