Add sample 4 to main readme

[TaoChenOSU]

Motivation and Context

The main README doesn't have a link to Sample 4 yet.

Description

Add a link to sample 4 with description to the main README.

Contribution Checklist

• [ ] The code builds clean without any errors or warnings
• [ ] The PR follows SK Contribution Guidelines (https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
• [ ] The code follows the .NET coding conventions (https://learn.microsoft.com/dotnet/csharp/fundamentals/coding-style/coding-conventions) verified with dotnet format
• [ ] All unit tests pass, and I have added new tests where possible
• [ ] I didn't break anyone :smile: