How to reduce the size of textarea's indentation?

I want to allow users to use TAB in my textarea to indent the text. The problem is the size of it. The indentation is too big (like 8) and I want to rezise it (to 4).

the way it is:

some text
        some text

the way I want it to be:

some text
    some text

I had no problem doing that for showing the text afterwards. However I couldn't do it at the editing time, in the textar开发者_开发知识库ea. thanks (:

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As BoltClock said, the CSS3 draft property tab-size should work. Firefox and Opera support the property with

-moz-tab-size: 4;
-o-tab-size: 4;

There's one guide here