Upload & Sell: Off
I shoot with a mark IV and have images at ISO 12,800 that were good enough for the intended usage. I have had images published that were shot at ISO 10,000 (in a newspaper...IQ not so important).
I have not used a IIn but many will say that the AF on that particular body was as good as AF gets. I love the AF on my IV but you may not be getting a huge increase in performance AF wise.
My guess is that you can shoot at least two stops over what you do now and be happy...meaning if you are okay at 800 on the IIn then 3200 should be fine to you.
The mark IV is a great camera. I am totally happy with it. It has become my do it all body. Weddings, sports, portrait....