In a mathematical system where 4=5 and vice versa, what does that mean? The only reason it seems weird to us is because we are taught what those symbols represent, so if the way we counted was "1 2 3 4 6 7 8 9 10" it would just mean our system was base 9. (or, rather, base "10")

It could also imply that the distance between values is either different to how we count things normally or it's inconsistent. Maybe in this numerical system, 2 = 2.5 and that is the only difference to our own.

It's all subjective to the majority of whoever agrees with it. Take PEMDAS for example. There is no rule in nature saying that parenthesis take priority over subtraction, and yet it is quite literally the basis for much of the math we use.

The only reason that all of the numbers we know don't equal each other is because we agree that all numbers are individual values, as opposed to equaling zero. Would it be stupid if there actually were a system where every number equaled zero?

To us, yes. But that's only because we haven't found a need for that kind of system in the common practice of mathematics. Who knows, maybe quantum physics will continue to screw with us and we eventually WILL find a practical use.

Until then, I guess it's interesting to think about? It is for me.