Four-Char Text CAPTCHA Builder

I need a lightweight text-based CAPTCHA that outputs exactly four characters, each character chosen at random from the usual mix of upper-case letters, lower-case letters, and digits. No images, no Google ReCAPTCHA—just pure text rendered on the page and validated server-side.

Here’s what I expect:

• A self-contained function, class, or small script that generates the four-character string and stores it securely for comparison.
• A quick demo page or snippet showing the CAPTCHA displayed and the user’s input being verified.
• Clear, step-by-step instructions so I can drop the code into my existing site with minimal setup.

Feel free to use PHP, Python, JavaScript, or any common web language; as long as the final code is clean, well-commented, and easy to integrate, I’m happy.