You say there is one twin. Twins, when referred to as a set, require an s, just like most other groups of objects.

There are the twins.
There is a twin.
There are the cats.
There is a cat.

And so on.

In the case of the four twins, you could say this:

Here come the four twins.

If you wanted to be more precise, you could say this:

Here come [the] two sets of twins.

Saying there are the twins does not imply any specific number, although many people would assume that there are two, since twins come in sets of two.

Remember that twins always come in pairs, i.e there are always two of them. So four people would be two pairs of twins. Two people would be a pair of twins. To say 'one twin' implies that you are talking about just one of the persons in a pair.

A "twin" is one of a pair of twins. One set of twins refer to two people who were born together. Several such pairs can be referred to as several sets of twins.

By definition twins are two individuals (in the case of creatures) born at the same time.

I hear people say things like "We have two twins in our family and both are identical boys." What two twins actually means it that there are 2 sets of twins, or 4 individual people.

When there is only one set of twins to say two before the word twin is redundant. (Unless of course there are 2 sets of twins)

So to answer your question twins are a pair or two things/people/animals. So you'd say,"This is my brother Matt. We're twins." If it's just Matt then he would say, "Hi I'm Matt. I'm a twin. My brother's name is Max. He'll be here later"

Twin has a couple of meanings:

• a set of two and only two identical things,

• a set of two people who were born at the same time, or

• a single person who is part of a twin

So twin in singular form will usually refer to a set of two, and two twins will mean four things or four people.

The third meaning is not very common unless someone is describing themselves or context heavily suggests the second meaning. But in that case, twins would refer to two people, not two sets of people.

Here is a very contrived example with four people (Alice, Bob, Charlie and Danielle):

Alice: Didn't I see someone exactly like you five minutes ago?

Bob: Yes, I'm a twin.

Alice: What's your brother's name?

Bob: My twin brother is named Charlie.

Alice: Hey Danielle, did you know Bob and Charlie are twins?

Danielle: No, I didn't.

2-sets -of-twins. twins being a two people and a set being a group of two. So a set of twins is 4. And I said 2 sets of twins so that’s 8.