I glassed both sides of a 4mm bulkhead with 4oz in a few s&g kayaks but haven't stood on them in the surf. My gut feeling is that a 4mm bulkhead glassed on both sides with 4oz with a clean fillet on the forward edge is sufficient,,,but short of actual testing we're mostly guessing until someone breaks though.

The only emperical evidence I can give for the strength of an unglassed fillet is an unglassed 6mm forward bulkhead put in a narrow four panel hull (Pax20)before the deck was put on.

It was put in the wrong location and a four lb maul was needed to remove it ripping out the top edge of the veneer in the side of the hull as multiple strikes along the edge were required, two dozen or so. I would think if it was held in by the deck an unglassed 4mm bulkhead would crack a lot before the fillet joint failed.

If you were anticipating standing on it as you strike the beach doing an endo and crashing onto your head I'd suggest glassing 6oz on both sides but mostly make sure that the fillet on the hull side was applied to a roughed up surface if epoxied/glassed.