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４．ZFCGについては、望月IUT IVでも取り上げられている
　そして、繰り返すが、21世紀の複雑化した数学では、一階述語論理に拘るのは拘るのは得策ではないと思う
　”一般的な圏論、つまり、意味論的な柔軟性をもち高階論理との親和性があるようなより現代的な普遍的代数が発展し、現在では数学全体を通して応用されている。”（下記）
　が、トレンドだと思うよ


(参考)
https://en.wikipedia.org/wiki/Foundations_of_mathematics
Foundations of mathematics
Contents
Toward resolution of the crisis
In practice, most mathematicians either do not work from axiomatic systems, or if they do, do not doubt the consistency of ZFC, generally their preferred axiomatic system. In most of mathematics as it is practiced, the incompleteness and paradoxes of the underlying formal theories never played a role anyway, and in those branches in which they do or whose formalization attempts would run the risk of forming inconsistent theories (such as logic and category theory), they may be treated carefully.
The development of category theory in the middle of the 20th century showed the usefulness of set theories guaranteeing the existence of larger classes than does ZFC, such as Von Neumann?Bernays?Godel set theory or Tarski?Grothendieck set theory, albeit that in very many cases the use of large cardinal axioms or Grothendieck universes is formally eliminable.
One goal of the reverse mathematics program is to identify whether there are areas of "core mathematics" in which foundational issues may again provoke a crisis.

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