Two plus two make/makes four.
What verb should I use there, make or makes?
Provide a reason also for your answer, please!
You will occasionally hear people employ plus as a conjunction, and even see people write it. But here it is unambiguously a mathematical operator, and the phrase two plus two is unambiguously a single mathematical expression.
You will find both of these used, but I think you'll find is used even more often than either make or makes. Here's an Ngram:
As for why both are used, that's not hard to figure out. If the subject is singular, the verb makes is used:
If the subject is plural, the verb make is used:
So, if the subject is two plus two, that could be considered singular or plural, depending on how you parse it:
Since either argument is a valid argument, you'll see it get used both ways:
And let's not forget: