Two and two (they) make four.

In addition to Kim's answer below. However, in mathematics, one would say that two plus two is four and two plus two equals four.

It's interesting, Kim uses a plural verb and Marco uses singular verbs. The possibilities are 2 + 2 make 4 (plural verb) 2 + 2 makes 4 (singular verb) 2 + 2 is 4 (singular) 2 + 2 are 4 (plural) 2 + 2 equals 4 (singular) 2 + 2 equal 4 (plural) I suspect that both singular and plural verbs are both used but that there is a regional preference. My personal preference (Australian) is for singular verbs, and I think this holds true for the UK as well. But it doesn't seem to me that plural verbs are incorrect. I have the feeling that in North America the preference is for plural verbs. Maybe I'm wrong, others might comment. So I think both are correct, but different people might tell you that one or the other is incorrect. I hope I'm being helpful.