Update README.md

Author: sylwia-budzynska

README.md

@@ -5,4 +5,4 @@ This repository contains challenges for the CodeQL Zero to Hero blog post series
 - [CodeQL zero to hero part 1: the fundamentals of static analysis for vulnerability research](https://github.blog/2023-03-31-codeql-zero-to-hero-part-1-the-fundamentals-of-static-analysis-for-vulnerability-research/). The challenges accompanying the blog post are in [folder 1.](https://github.com/GitHubSecurityLab/codeql-zero-to-hero/tree/main/1)
 - [CodeQL zero to hero part 2: getting started with CodeQL](https://github.blog/2023-06-15-codeql-zero-to-hero-part-2-getting-started-with-codeql/). The challenges accompanying the blog post are in [folder 2.](https://github.com/GitHubSecurityLab/codeql-zero-to-hero/tree/main/2)
 - [CodeQL zero to hero part 3: security research](https://github.blog/2024-04-29-codeql-zero-to-hero-part-3-security-research-with-codeql/). The challenges accompanying the blog post are in [folder 3](https://github.com/GitHubSecurityLab/codeql-zero-to-hero/tree/main/3).
-- [CodeQL zero to hero part 4: Gradio case study](). The challenges accompanying the blog post are in [folder 3](https://github.com/GitHubSecurityLab/codeql-zero-to-hero/tree/main/3).
+- [CodeQL zero to hero part 4: Gradio case study](https://github.blog/security/vulnerability-research/codeql-zero-to-hero-part-4-gradio-framework-case-study/). The challenges accompanying the blog post are in [folder 4](https://github.com/GitHubSecurityLab/codeql-zero-to-hero/tree/main/4).