Patient advocacy is an ethical nursing practice, but its valid and reliable measurement remains problematic. The majority of existing scales lack robust psychometric evidence in diverse clinical environments. This study aimed to develop and psychometrically validate the Nursing Advocacy Scale (NAS-4), which is a multifaceted scale to measure challenges, strategies, and influencing factors on patient advocacy. The research was cross-sectional with 148 nurses. NAS-4 was applied, and the factorial structure of the NAS-4 was examined with Exploratory Factor Analysis (EFA). Internal consistency was assessed with Cronbach's alpha, and construct validity was assessed with known-groups validity and convergent validity analysis. EFA affirmed a four-factor solution for the core advocacy construct: 'Operational & Systemic Challenges,' 'Professional & Ethical Challenges,' 'Interpersonal & Social Challenges,' and 'Advocacy Action & Competency.' The overall scale and its subscales had high internal consistency (α = 0.78 to 0.92). Descriptive analysis of the items revealed that nurses strongly agreed that factors like supply and demand (M=4.49, SD=0.71) and ensuring patient safety (M=4.61, SD=0.75) were mandatory, while fear of losing one's job (M=3.13, SD=1.30) was a neutral challenge. Construct validity was established, with the scale differentiating well between groups with experience and training. The NAS-4 is a psychometrically sound instrument with proven reliability and validity for the measurement of the multifaceted construct of nursing advocacy. It is a valuable tool for health organizations and researchers to employ in the identification of barriers, audit of advocacy competency, and the development of effective targeted interventions to facilitate nurses as patient advocates.