4mm seems a bit thin of material, but as for a marine ply requirement, there really isn't one. I use 3/8" or 1/2"( 10-13mm) outdoor plywood, then use standard fiberglass resin with approximately 10-15% acetone to thin the resin. Then just paint it on the bare wood as a sealant. Good paint may work as well, but I've had good success with resin and acetone. Use some thin foam and spray adhesive, then staple the vinyl on. If you leave the vinyl in the sun, it stretches much better.