What could you code in four hours?

I would probably go the highest-level route I could, using tools that I am familiar with (and won't get caught in nasty workarounds due to my unfamiliarity with the platform).

In my case right now, I'd probably put together a web mashup using a container of useful platforms--Like Apache ServiceMix using Camel, ActiveMQ, CXF, etc. to grab services, munge them, and output something useful.

Then again, going super low-level might be fun too. Go to the depths of the OS or a compiler and fix an annoying persistent issue.

I would think the most important thing to do first is to stop and think about the logic of the code. Plan a small and simple roadmap in your mind, or on paper if your mental memory is low.

This will save you a lot of the try-and-fail attempts, and even better, before you write a single line of code, you have a general idea of what it is that you're doing and how it is that you're going to be doing it.

I'd write a programming language that targets OCaml.