Thanks, Marius. Yes, I know it's because we are talking about a period. What I wanted to know is whether there is a name for this. Can we call "two years" a collective noun? It doesn't look like a collective noun because collective nouns do not have singular/plural forms, e.g. the audience, the cabinet.
For the second one, the meaning of the sentence is pretty clear. I don't need four men, maybe just three. I inclined to say, "four men is too many", but I'm not too sure.