According to the upgrade instructions, you'd need to go to 2.2.x before you can upgrade higher than that. That's not to say that a course backup from 1.9.x wouldn't work if imported into 2.4. I think you'd just need to try it and see. It probably would depend on what types of activities you have in your courses.
From what I can gather from recent postings, your safest upgrade path is probably 1.9.9 ==> 1.9.19+ ==> 2.2.6+ ==> 2.3.3+
You could, of course, go right to 2.4+ from 2.2.6+ realizing that you'd be on the bleeding edge of Moodle. We did that with 2.3 and had far too many issues. So we'll probably wait until 2.5 is close before going to 2.4 just to try to be safer about it.
The jump to 1.9.19+ is important because changes were made in the later 1.9.x line to make upgrading to 2.x smoother.