Concrete is made up of three main ingredients: water, Portland cement, and aggregates. The ratio of the ingredients changes the properties of the final product, which allows the engineer to design concrete that meets specific needs.
I don't know how one could estimate the chance of structural failure between 3000 vs 4000 psi. I believe there are many other variables that would/could impact the long term structural soundness of a concrete structure. For example: sub grade conditions, steel rebar layout/design, temperature during placement, placement procedures, curing conditions, climate and freeze /thaw cycles, thickness, slump properties, etc. That being said, If I had the choice I would always choose the higher psi mix design for any water containing vessels, retaining walls, light commercial slab applications, etc. ; I am not in anyway an expert on concrete or pools though.