<p>Cultivating students' geometric proving skills has a significant impact on their development of mathematical competence. The van Hiele model of geometric thinking and the Worked-out Examples Involving 4 stages (WEI4S) instructional approach have been proven to improve students' geometric proving skills effectively. A quasi-experimental study with 78 ninth-grade students (38 in the experimental group and 40 in the control group) learning inscribed quadrilaterals was conducted to evaluate the feasibility and effectiveness of using the van Hiele model and the WEI4S instructional approach in teaching geometry in ninth-grade mathematics in Vietnam, aiming to improve academic performance and cultivate students' geometric proving skills. Pre- and post-tests, surveys, and classroom observations were used to collect research data. While data from pre- and post-tests were analyzed quantitatively by SPSS software version 22 with tests such as Pearson's correlation, independent and paired-samples t-tests, the Wilcoxon signed-rank test, and the Stuart–Maxwell test, data from surveys and classroom observations were analyzed descriptively and qualitatively. Results show that the experimental group achieved better results than the control group (p &lt; 0.05) and their performance improved significantly after the intervention (p &lt; 0.05, Cohen's d = 1.77). The analysis indicates that using the van Hiele model and the WEI4S instructional approach in mathematics teaching improves student learning outcomes, develops geometric proving skills, and increases the experimental group's participation in explaining and justifying mathematical problems. Additionally, this study's limitations include a small sample size, a short experimental period, and a limited scope of impact. The study also represents implications for mathematics education and mathematics teacher training programs, as well as recommendations for new research directions.</p>