Does the word "for" make any difference between the following sentences:
I've been in France four months.
I've been in France for four months.
Hello, D.T.—No it doesn't make any difference whether you add "for"; the meaning will be the same either way. Incidentally, the sentence is ambiguous. It could mean either that you have been in France (for) the last for months or that it is true that, at some point, you have been in France (for) four months. So, if you are talking about the last four months, it would be good to add "the last."