Add a character(TAB) to begin of selected lines

1. Write a loop that will go through the text.
2. Every time you detect a new line character you will have to add a "\t" in that index.

Try this to get started:

private void button4_Click(object sender, EventArgs e)
    {
        // get the selected text
        string selText = allText.Substring(richTextBox1.SelectionStart, richTextBox1.SelectionLength);

        // get the collection of Lines
        string[] rtfLines = richTextBox1.Lines;

        string theLine;

        for (int i = 0; i < rtfLines.Length; i++)
        {
            // compare each line
            theLine = rtfLines[i];

            // to see if it's in the selection
            if (selText.Contains(theLine))
            {
                // in selection : add a tab
                rtfLines[i] = "\t" + theLine;
            }
        }

        // reset the content of the RichTextBox
        // this necessary because changing a line in the
        // collection of Lines does not affect the RTf content
        richTextBox1.Lines = rtfLines;
    }
}

Note this only will add a tab at the start of lines that are fully contained in the current selection. If you have a partial selection in a line, that will be ignored.

Tabbing in a partial selection could be achieved, but that's another chapter: perhaps one you will write ?