If you want something in PU perhaps a 4x6x4 could be the answer...
However there are many options in today's market:
1. There are products like this which may be worth looking into:
inlayz.net/strength-tests 2. Epoxy resin with Extruded Polystyrene Core (XPS) will allow you to utilise a heavier fiberglass cloth, but with similar weight to your traditional 4+4x4 PU board. However with heavier cloth comes a loss of flex - which is perhaps what you are noticing with that lighter board you are using, rather than the weight of the board itself.
3. Expanded Polystyrene Core (EPS) construction with laminates etc. No degradation of flex like in PU.
www.joshdowlingshape.com/techone/techone.html katanasurf.com/technology/