Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease worldwide, and its prevalence continues to rise. Fibrosis-4 index (FIB-4) has been shown to be a prognostic marker of liver-related outcomes in patients with NAFLD. We analyzed data from TriNetX global federated research network, combining data on 30 million patients. Patients were categorized into three diagnostic groups: NAFLD, nonalcoholic steatohepatitis (NASH), and at risk of NASH. Primary outcome was all-cause mortality, and secondary outcomes included progression to NASH, development of cirrhosis, end-stage liver disease, hepatocellular carcinoma (HCC), and liver transplantation. A total of 442,277 subjects (1.5% of the cohort) were assessed, and 81,108 were retained for analysis. Median follow-up was 34.8 months (interquartile range 12.2). FIB-4 was < 1.3 in 52.3% patients and ≥ 2.67 in 11.4% patients. In multivariate analysis, FIB-4 ≥ 2.67 was significantly and independently associated with all-cause mortality (hazard ratio [HR] 2.49, 95% confidence interval [CI] 2.20-2.82, P < 0.001) as well as with progression to NASH (HR 5.78, 95% CI 4.72-7.07, P < 0.001), cirrhosis (HR 2.04, 95% CI 1.86-2.24, P < 0.001), end-stage liver disease (HR 1.86, 95% CI 1.68-2.05, P < 0.001), HCC (HR 3.66, 95% CI 2.71-4.94, P < 0.001), and liver transplantation (HR 7.98, 95% CI 4.62-13.79, P < 0.001). Conclusion: In a real-world nationwide database, FIB-4 ≥ 2.67 was a strong predictor of both all-cause mortality and liver-related adverse outcomes independently of the baseline diagnostic group and common risk factors. Our findings indicate that FIB-4 could play a role as a risk-stratification tool for a population health approach. Significant underdiagnosis of both NAFLD/NASH and NASH cirrhosis in electronic medical records was observed.