Strength, as in bending stress and defection, depends on the moment of inertia (a function of shape of the cross section) and the modulus of elasticity (a function of the material). There are other factors such as impact strength, whether the material strength is the same in compression or tension (concrete is not good in tension, only compression) and elongation.

Back to comparative strength. The moment of inertia is a reasonable measure of comparative strength. The moment of inertia for square and rectangular cross sections is proportional to the cube of the thickness. 3 cubed is 27; 4 cubed is 64. Divide 64 by 27 and you get 2.37. Comparatively, the 4 mm is 2.37 times stronger than the 3 mm. This is for the wood only. The fiberglass makes it a composite structure that is thicker than the wood, but you can still go with comparing the relative thicknesses of the section.

Whether this is important in the real world is a decision you have to make. You can make all the calculations you want, but it only gets you into the ballpark.