depends,,if the boat is in a hot setting for a stretch of time the air will find a way out,,drop it in cold water and I can guarantee you will see the decks go concave in spots. I don't know about the reverse. A glassed 4mm deck is pretty strong but there's a fair amount of pressure difference with 30-40 degrees sudden change. I saw a 3mm Pax 20 deck implode a dime size hole near the bulkead that was sealed as you have when it went from hot sunlight to cool water it sounded like a bang. I doubt if the 4mm deck would do that. If you haven't sealed the bulkhead at the deck nothing will happen but neither will it be waterproof.
But mostly it's kind of funny how a "default" decision to incomplete the construction,,leaves an incomplete construction. Bulkheads go with hatches,,no bulkheads, no hathches/drybags.