4mm will work and so will 3mm. IF it's quality BS1088 okoume. I have a kayak that is entirely 3mm ply that's been around for 10 years and it isn't even covered with glass.
You can build kayaks from the cheap stuff but it usually turns out to be too flimsy, if unglassed, to guarantee a moderate life span and it may not tolerate anything more turbulent than a farm pond.
The first thing you want to know is if the middle veneer has any gaps in it. Gaps cause structural weaknesses. You can scan a sheet of thin ply with a 250 watt spotlight applied in a dark room. The voids will show up as an orange glow. You can test for waterproof glue by boiling a piece. It should last for an hour without delaminating.
Or you can use your head and pay the price for marine quality ply. $20 a sheet ply is never going to be as good as $70 a sheet goods.
A hull for someone your size has to be bigger than one for someone 100 pounds lighter but, otherwise, everything else is about the same.

: 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.
: It looks relatively knot free on the exposed surfaces which also happen to
: look quite ugly. My question is should i even consider using this stuff,
: it is around 3 times cheaper than high quality marine or exterior grade
: plywood of the same thickness. If anyone here is from australia, its
: bunnings plywood in 4mm.

: My 2nd queston, is 4mm plywood really thick enough? I have no doubt that for
: the general hull this should be more than adequate, but im curious how it
: will handle my entire body weight on one spot when i step one foot into it
: as i get into it off a jetty (i weigh about 98kg ~ 240lb?)