From C-h v default-tab-width RET
This variable is obsolete since 23.2; use `tab-width' instead.
and for tab-width
Documentation:
*Distance between tab stops (for display of tab characters), in columns.
You can customize this variable.
When I insert
(custom-set-variables
'(tab-width 4))
into ~/.emacs
and restart emacs, it is set to 4
.