Not Quite Four but that is not the point...

Four.py

from __future__ import annotations
from dataclasses import dataclass


@dataclass
class NonZero:
    succ : Nat

Nat = NonZero | None
Zero = None

def add(x : Nat, y : Nat) -> Nat:
    match x:
        case None:
            return y
        case NonZero(succ):
            return NonZero(add(succ,y))

if __name__ == "__main__":
    print(add(NonZero(NonZero(Zero)), NonZero(NonZero(Zero))))

REPL.txt

$ python3 Nat.py
NonZero(succ=NonZero(succ=NonZero(succ=NonZero(succ=None))))