I've had exactly the same problem with OS4 on iPhone. It would not let me in, even though the login details were correct. I tried removing the app entirely and reinstalling it, to no avail. However, when I looked at the Facebook help pages to change email account - thinking that might help - I found out that you can log in from any email account you have registered on Facebook. I used a different one and it let me in. Just add another email account if you only have one, or switch to another and that should solve the problem.