Ask HN: In Subversion, how best to set up a new major version of my application.

I'm about to start work on a new version (version 4) of my commercial application. I use Subversion.

Based on your experiences, mistakes, and successes, how would you recommend I set up the new version in Subversion?

Here's some info: I intend to keep releasing critical updates in version 3 for some time after version 4 is released. However all development of new features will be solely in version 4.

In case it is relevant: I'm a solo developer on this product, and that is likely to remain the case.