depends,,,,on where that 3,4mmply is being utilized. For flat panels 4oz on both sides of 3mm is very flexible,,but might be fine on a paneled deck where there's an extra layer along the joint and the panels are narrow. For a hull bottom panel in a four panel hull glass on one side of the bottom panel won't be sufficient if the boat gets much use as the interior flexes up enough to open little micro cracks for water staining.
it depends.