I am in fourth year and didn't get to know any of my professors really well. Is it too late to ask for letters of recommendation for graduate school?
No it's not too late, but you're going to be disadvantaged - mainly because your letter writers probably cannot say much about you that your transcript doesn't already say. They might be able to comment on your coursework performance, but cannot say anything about your research ability, which is going to have a negative impact.
If you're interested in graduate school and can still do some research project, you should take the opportunity.