You have a GS 3......then don't bother changing!
Buy a rock quicksand case for it, makes the phone look better, plus adds a lot more grip.
The camera looks average in comparison to the GS 3 for most scenarios (there are some shots where it looks just as good but on the whole there is no doubt the GS 3 is better) especially the 1080P recording. The camera is better than the GN and some people who came from the GS 2, have said it is just as good and others that it is better.
Audio isn't as good, especially the loudspeaker, however, with a good set of earphones, I doubt you will notice a huge difference between them unless you're an audiophile
16GB isn't really a problem unless you intend on keeping your entire music collection on the phone, on my S (10GB for videos, photos etc. and 2.21GB for apps) I still have 3GB for videos etc. and 1.28GB for apps and this is with over 1000 photos including 1080P and slow motion recordings, wallpapers, icon set, N64, genesis ROM emulator files etc. and over 80 apps installed including a number of games like dead trigger, dead space, angry birds SW, where's my perry/water etc. and I just use google music for my 2GB of music

I could easily clear about 5GB up too as I backup all my photos/videos to my PC daily.
At the minute, the battery isn't as good as the GS 3, however, the battery drain on the N4 is due to poor software optimisation, which should be fixed in a few updates and once it is, in theory it should be better than it. When will that fix happen, no idea, could be a few days, weeks or months......
You would be downgrading 3 out of the 5 factors you mentioned if you went from the GS 3 to the N4.