Is This True? [Slightly Off-Topic]

To elaborate little, people usually consider the elements of Z_4 to be equivalence classes of integers.
For example, in Z_4, [0] is the “integers equivalent to 0 mod 4”. So in Z_4, [0]=[4]=[-4]…

If you think of the elements of Z_4 as ‘0-3 with special rules’ then ‘4’ obviously doesn’t make sense.

I haven’t got to rings, but my teacher basically taught us that 4 technically isn’t in the set, but I see what you mean by equivalence.

@warrior, are you referring to relatively prime? Or a different concept?

In the sense that you’re used to “0” isn’t part of Z_4 either.

Regarding primes…
http://mathworld.wolfram.com/PrimeNumber.html
http://mathworld.wolfram.com/PrimeElement.html