I'm hesitant to use "reductionism" because I already interpret that to be a belief about the material world (747s made of quarks and so on), not about propositions. I know people who accept material reductionism, but not propositional reductionism.

The real positivists were willing to accept that 2+2=4 was irreducible, since they considered it a tautology/definition and so exempt from testing. I am split: I think in one sense it's tautological, but that we pay attention to that particular tautology for reasons involving a testable generalization over all cases where two objects have been added to two objects and the result has been four objects.