Voted for the 400mm F4. I think this is very potential lens now they have the glass and parts for the 200-400 which it could share a lot of. Just a simple matter of a few adjustment and voila! Although I'm not so sure this would fit into my reasonable big white price thinking of £4500-£5000 judging on the price of the 200-400.
I think your price is in the right ball park. The cost should be slightly lower than the 300mm f2.8L USM II IS.