Ask HN: Adding tests to an established code base?

I'm not entirely sure how to phrase this question because there are probably a lot of different ways of asking this based on the nuances but here's my particular situation:

I started a project a while back. It wasn't much of anything when it began but now it's big. The last few pieces I built actually got some tests to cover the basic functionality though they are likely far from thorough (though, it is amazing what those tests did expose in terms of bugs!). So here's my question...

Now that thing is actually starting to be something of a formidable code base, what should I do with the rest of the code that doesn't have tests (probably 80-90% of the code base)? Here are my options as I see them. Am I missing anything?

1. Write tests as I find and fix bugs.

2. Rewrite all of the untested features and add tests as they get rewritten.

3. Just ignore the tests.

4. Add tests to the backlog and work on them as time/budget allows until they are all written.

What does HN suggest? Rewriting? Backlog? Should I quit my job and become a goat farmer?