Full Idea
There are two ways to read to read '2 + 2 = 4', as singular ('two and two is four'), and as plural ('two and two are four').
Gist of Idea
'2 + 2 = 4' can be read as either singular or plural
Source
Thomas Hofweber (Number Determiners, Numbers, Arithmetic [2005], §4.1)
Book Reference
-: 'Philosophical Review 114' [Phil Review 2005], p.194
A Reaction
Hofweber doesn't notice that this phenomenon occurs elsewhere in English. 'The team is playing well', or 'the team are splitting up'; it simply depends whether you are holding the group in though as an entity, or as individuals. Important for numbers.