Hi guys I'm 4 yoe working in react and spring boot but I want to add additional skill what should I start to learn

I have no clue what else you already know. But you can try as per your interest from below.

1. Cloud services - AWS/GCP/Azure
2. DevOps - Docker, Kubernetes, Jenkins
3. Database - SQL, NoSQL
4. Mobile development
5. Cybersecurity
6. Machine Learning