For the most natural way, I would say it like this: It has been 10 years since we last saw each other. (But in my sentence, and in any of the sentences, we do not know if the two people have seen each other again; that is, after 10 years.) To comment on your sentences: 1) This is correct grammar. But it is not as natural as my sentence. 2) If you just insert "last" as I did, then this sentence becomes great! ;) 3) & 4) I personally would not say these sentences. It sounds awkward to use the present tense ("It is 10 years") in this manner. This is why the present perfect ("has been") is used: to refer to something that started in the past that continues until the present moment. I think a more common way to use the present tense in such a manner would be as follows: That makes 10 years since we have seen each other. That makes 10 years since we last saw each other. But in these two sentences, the two people have still not seen each other again.

I believe your first sentence would be the most common way to phrase it. The second sentence is also grammatically correct, but is a little more awkward than what you would hear in casual conversation.

2) It has been 10 years since we saw each other. This is correct and good and useful. It does not need any other phrase. But see my answer to express it more naturally by adding one word to it.

In case you mean you haven't seen the person you're talking about since 10 years ago, you can use number 2, you just need to add something, as you said, for example: It has been 10 years since we last saw each other. or It has been 10 years since we saw each other in this coffee shop. I think the phrase is not correct if you don't add a complement but I'm not sure. For number one you have to change the second part of the sentence It has been 10 years since we haven't seen each other. Number 3 and 4 are not correct, but I don't know how to explain why. I think it's because of the time it is written in.

I think No.1 is the most usually used phrase. :D