Thanks for reply!
I have more or less realised that 2K will be too much to ask for at 4000fps. 1920 might be the one in the end.
The thing is that we are finishing in Lustre and my experience is that Lustre can compress 35mm scanned to 2K into 1920 and 720 very well and the grading looks fabulous but when starting with like 720 then it really doesn't hold.
Therefore I would say that 1920 is my rock bottom.
The action we are shooting are explosions, kind of like the end of Zabriskie Point but the director is talking about take it further. We are also going to shoot some things in 1000fps, like close-ups with differnet kind of liquids running down the face.
Honestly I have not much experience with "raw" HD, 90% of what I do is 35 but as I understand it that shooting at this speeds digital is better. If you have any opinions in this matter, I would really appriciate!
You seem to be much more experienced than me shooting high-speed, Im very thankful for all your input!