Upgrade D3 to D4 or D4s?
Is a used D4 a worthwhile upgrade to my D3?
Yes, but the D4S would be more perfect for you.
I am looking for a bigger buffer, a few more pixels and better ISO performance. I understand that some are saying that the move from a D3s to a D4 is a backwards step when it comes to ISO.
I'd say that's not at all true, little discernible gain but clearly not a loss.
Should I rather wait and save a bit more and go for the D4s?
It's up to you but I would wait. I traded my D3S for a D4S and to me it's very much worth the upgrade just in the way it handles noise at high ISO and the white balance is better. Not to mention that you'll never bury the buffer on the 4S.
A local guy is asking $4500 for a D4 with 50,000 actuations whereas the price of a new D4s is around $8500 here in SA.
That's a big difference in price so I see your dilemma, however, I'd hold out for the 4S.