I did this once.
Didn't have to change the nut; did have to change the setup.
Totally hated not having a G sting.
Bought a 5 string.
Then: quit playing 4 strings.
My advice would be buy a 5 string and get on with it.
I just can't afford it yet, but a 2014 5 string fender deluxe is going to be my next purchase.
Thanks for the advice everyone. I'll report back soon on my findings.