Is there a good guide for going from 1.4 to 1.6?
Just take a look at the backward incompatible changes in both sets of release notes to get an idea of how the migration will affect you. There are lots and lots of nice improvements in 1.6 and especially 1.7 that you may be interested in.
https://docs.djangoproject.com/en/dev/releases/1.5/
Also, remember that when 1.7 is released, 1.4 will no longer received security updates, only 1.5 and 1.6 will.
However, this is only security updates, not necessarily bug fixes.
[1] https://docs.djangoproject.com/en/dev/internals/release-proc...
Having recently upgraded two production projects from Django 1.3 to 1.6, I'd say YMMV but it likely isn't. Go public unless you have someone at hand with enough eagerness and knowledge of the codebase to do the migration quickly so you don't have to postpone the release.
The migration itself is no big deal, as others pointed out—you just make a checklist of backwards-incompatible changes and walk/grep through the codebase for every item. I don't think any DB schema migrations were required. Took me two days a project, not counting manual testing afterwards (unit tests were there, which is convenient but can't guarantee that everything will work).
The process is not very exciting, though. It helps when codebase is stable and you have the time to go through it without rush and make sure everything works for some time after deploying the upgrade to production (being ready to roll back quickly). That's also why I'd suggest going public and leaving Django upgrade for later, when things calm down a bit.
1. How many 3rd party apps are you using? Be especially wary if the a) perform any clever black magic on Django internals or b) are only lightly maintained and the job of updating them might fall to you.
2. Do you have a decent test suite?
Beyond that it was pretty straight forward, but it really helps if you have good unittests to begin with.
Do the upgrade. Just read the release notes for every release between the versions you are going through and make sure to make the changes they suggest/require. It'll take you 2 hours and yes it is worthwhile.