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I got the error. I am using Python 2.3.4.
Hitting TAB in the editor advances the cursor by "4" spaces
irrespective of the "indent width" in Configure idle...->
Fonts/Tab  -> choose indentation size:

I did a quick and dirty fix.
I had to modify the Line# 915 in file <py install
path>Python23/Lib/idlelib/EditorWindow.py

Requires restart of IDLE for the change to take effect.

[Before]
    indentwidth = 3
[After]
    indentwidth =
idleConf.GetOption('main','Indent','num-spaces',
                default=4,type='int')