1. "c'est un" with liaison is formal, we don't do the liaison in everyday conversations.
As for the numbers, it's simple, there's no liaison. When the word itself is pronounced with a "t", we pronounce it. We don't pronounce the "t" in the words "cent" and "quatre-vingts". But we do pronounce the "t" in the word "vingt". That's why you got confused,
2. Because the "t" is almost never pronounced at the end of a word.
3. quatre-cent-vingt-et-un
4. I don't get what you mean. It's pronounced [katʁ].