Go to
Options > Document Options
1) Set Tab width to 4
2) Select Expand Tabs
Tab Width : The width of a tab character in character spaces. Above this is set to 4.
Expand tabs : If checked then Source Insight will expand a tab character to the equivalent number of spaces when you type a tab. The text will look the same as if a tab was typed, but spaces will be used to fill. If not checked then Source Insight will simply insert a tab character into the file when you type a tab.
References :-