issues with Terminal transparent Background

There are two 'Opacity' sliders in your pic.

Maybe the one you need to change is the upper?

In addition, your 4% setting appears to be for 'inactive windows'. As soon as you click a window it becomes an 'active' window with a different opacity setting.

When you sort this out, you'll also want to click in the menu bar on Shell > Use Settings as Default so that these remain the next time you launch Terminal app.

Apple > System Settings > Accessibility > Display > turn on Reduce transparency

Plus your user profile shows that you are running macOS 15.1 instead of the latest 15.2.

What's new in the updates for macOS Sequoia - Apple Support

• but when I click on it background becomes black event though I have changed opacity to 4%

The opacity you refer to applies to inactive windows, and there are none shown in your second screenshot. That screenshot shows an active window with an opacity of 100%. It appears to be correct.

"Minimizing" the Terminal window or selecting another Terminal window makes the former inactive. Your first screenshot also appears to illustrate correct behavior.

Your settings are doing what they should.

You’re welcome. 🙂