: I have readily available dodgy quality 4mm plywood sheets (2440x1200mm) for
: around $20 australian a sheet to build a S&G guillemot. This plywood is
: completely random, i have no idea what grade it is or anything like that.

You can use non-marine plywood BUT make sure everything is sealed really well, and keep a close eye on scratches of the hull. Also, apply epoxy filler to all S&G holes since they suck up a lot of epoxy which can compromise the seal. Ask yourself if the $100.- you save is worth the potential risk of water leaking in, especially since you will be spending $300.- on glass and epoxy. And of course check for voids etcetera, don't use total junk. Weigh the plywood too, there is a big weight difference between soft and hard plywood. Yes 4 mm is certainly thick enough.

Henk