FindControl Question

This is my loop to find a particular textbox in a gridview:

dim selectedvalue as string = "hello"
For i = 0 To GridView1.Rows.CountDim tb As TextBox = CType(GridView1.Rows(i).Cells(4).FindControl("textbox4"), TextBox)   --- cells(4) is the 5th column in the gridview
tb.Text = selectedValue  -- selec ...