"Four people work here."
Translation:Здесь работают четыре человека.
This is one of that crazy grammar rules which is extremely difficult to explain.
In a few words, yes, the verb can be either a singular or plural depending on speaker's preference.
I wish I would find the article about this in English, but I can't. Here is the Russian version, however: http://new.gramota.ru/spravka/letters/22-spravka/letters/64-bolshinstvo
Hmm, I see.. I don't really understand that page, but from what I gather, I think it's because it's "Four 'of A person' or of A group of persons", which allows conjugation of the singular group rather than the plural four ?
Like "four people work here/A group of people comprised of four units works here" ?
Yes, I think you got it right. We may consider four men a group of people:
Здесь работают четыре человека. (plural)
Здесь работает группа из четырёх человек. (feminine noun)
Also, "миллион" is a masculine noun in Russian:
Здесь работают миллион человек. (number)
Здесь работает миллион человек. (noun)
The adverbs "много", "мало", "столько" look like a neuter ("-о"):
Здесь работают много человек. (rarely used)
Здесь работает много человек. (preferable)
In case you haven't found an answer to your question yet, here's the explanation: numbers in Russian behave very differently than they do in English. A number that ends with one of these words [два / три / четыре] calls for the noun it quantifies (and all adjectives that may be attached to it) to be declined in the genitive singular (nom. singular человек -> gen. singular человека). Check out the lesson "Time and Numbers" for a more fleshed out explanation!
I do have bad news for you though; человек does have a plural form that isn't люди. I believe it is used when the number of "people" it refers to is known, or in general when the word "people" isn't used as an unquantifiable. Don't take my word for it though, I haven't quite gotten the hang of that particular subtlety myself.