Hi. I taught myself Ruby and Rails, and have some basic PHP and javascript, but wouldn't be confident in the latter two. Am I just going to get in the way at a Hackathon?
No, you won't just get in the way. Hackathons just aren't that serious - they are essentially a more organized venue for a particular hobby. Go ahead and go. You will likely learn something. As long as you're honest about what you know and what you're capable of, the worst thing that can happen is that an asshole will be an asshole, which they probably would be anyway.
The first hackathon I attended I was a beginner programmer - but was welcomed and found it a great learning experience. Many experienced developers like the eye of a younger coder to help them rethink an idea rather than to do it the way they've been doing it for years. Also, there is more than just coding to be done!