Why is it ppl keeping asking for nigh-impossible things in threads like this?
a 400mm f/4L IS lens is not possible at $2500!
(The weight reduction in the OP is also a little optimistic, too. I understand that this stat was based on the improvements made to the new super400, but that was 28% weight reduction on one of the very heaviest lensses going around...)
I had to choose between a 300 and a 400 when I was getting lenses, and I go tthe 300 because it had IS. If a 400 came out with new optics, new IS & weather sealing, I'd be happy to pay an extra $700 over the current retail price of the existing model. I'd probably still get one if it was $1000 over the retail price of the current model, but I wouldn't be as happy.
I really like Gareth's suggestion of shortening the minimum facus distance too. I hadn't thought of that.