To do this, put an unbound textbox on your Report. I'll call it TextSum. Set the Control Source to =1, set the running sum property to Over All, and set the visible property to No. Then add the line, I'll call Line10 just below your textboxes and set the Visible property to No. In the Format event for your Detail Section put this code. Make the necessary adjustments to the control names.
If TextSum Mod 4 = 0 Then
Me.Line10.Visible = Yes
Else
Me.Line10.Visible = No
End If