I do not exspect it to work, sorry, but the 4mm deck will not last. For the bottom it will behave like an eardrum (hope it is the right word, acc. to my dictionary this is not only the anatomical eardrum, but also works for audio)
meaning it will work, as long as you do not hit anything.
I cannot comment on your plywood frame, I have never worked with a plywood frame, i use balsa only, but you can test it easily by yourself, put a plank to distribute the weight and then add weights, or better stand on it. If this lasts, it will last…
The deck, especially the later feet, knee and hand area, will bent down, it may spring back, but it will be a weak spot. The area betwenn stringer and rail, between to ribs is not strong enough.
But good news, you can always add additional layers of glass.
But I would suggest another way: Laminated at least the deck (bottom optional) from the inside too, a 3oz cloth should be enough, laminating in and outside results in an H-Beam; the higher the better. So if you will use 6mm balsa for the deck, it will be even better. Laminating the inside can eliminate the need for the second glass layer on top, so it still will be light.
How it works: Prepare your deck, apply a cheating layer of resin, very thin (on the later inside!) This will close the pores of the balsa and prehibits that to much resin will be sucked away from the following glassing. After fully cured, glass the inside, using a fair amount of resin, better a little more than usuall, the glass shout be really wet!
Let it sit until tacky, but still not cured. In the meantime prepare thickened epoxy as a glue and apply it onto your frame, then glue the deck onto your frame (still tacky inside). You definitely need to support the rocker or better use a rocker table. Do if wanted, the same with the bottom.
You may check some of my builds in detail (and my mistakes and faults) on www.woodboardforum.com, username: olddude.
A first impression of my last build, you can find here on swaylocks, searching single fin 6’6
