Four wires makes perfect sense and I would twist each pair together for aesthetic reasons. It will make no difference to the noise levels as the amp shown has no high gain parts near the cables.
I use double insulated wire here because it looks more beautifull and professional. If I have two wires w/o double insulation - I twist them and sometimes put into shining soft-silikone type termal-shrinkable insulation. It looks definitely better.