3
I would go through release notes for 1.4, especially the backwards incompatible changes, noting everything that is new in 1.4 and is being used in your project, and has to be changed first hand.
Then, if you have a test suite (which will help a lot in this case) run it against 1.3. If not, well, get at least minimal coverage, basically GET all your views in it, and run against 1.3.
Else, like Ignacio said, just plug and pray 
4
Why are you not using virtualenv?
This would allow you to run this app with its own copy of django (1.4) without troubling the rest of the server.
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