I wouldnt worry too much, 4 mm its plenty, eventually if it really is a thought that sticks on the back of your mind you can add an additional layer of glass to the bottom only.

As for the look just paint it, usually people wants to show the wood but you know what? there are amazing looking kayaks out there that were painted over because of the wood wasnt that nice, dont go crazy filling all the grain if youll glass over and paint. You can still get some nicer wood for bright accents like coaming and additional rubbing strips and stems.
If you paint it try to avoid dark colors they can get extremely hot under the sun.

For the quality youll get tons of people telling you that for the work and money involved better get the best etc, point is that theres lots of guys out there enjoying their doorskins boats painted with exterior house paint, and those things if mantained can last years. After all a kayak isnt something that will stay in the water 24/7. The fact that you arent sure about liking the stability for fishing (its after all a little subjective) and that the kayak could be modified after some trial its something that adds to "make the best use of what you have".
I would still, though, check carefully the edges for voids, wont guarantee that there arent any through the whole sheet but its an indication of plywood quality too, if you can spot a few voids on the edges youll have voids inside, and cutting the panels will reveal them. Number of knots and eventual knots cut out and filled with wood diamonds or filler would give you an indication of grade too depending on the standards of your country.
I would even do a little boil and dry heat test, theres some plywood that has some glue so poor you could bet its flour glue and just the sun pulls it apart or makes bubbles, thats the stuff you want to avoid at any cost. Other than that remember that you are still in time to build your "perfect" hull after.