What’s the best codec to use for PC win 7 and mac for editing applications?
I’m using Nuke, After Effects and Premiere.
Free or cheep is good also alpha channel support is a must.
I like apple prores But you can only view it on Pc not export in that format.
4K support is a must as well.
Hmm…I am a little confused about the question. Are you asking what formats others are using in production? Or are you asking about quick time specifically? I think that the answer will very depending on what you are trying to accomplish and how you are getting your footage. Regardless though, it is probably not a good idea to compress your footage until final output if at all possible. So any format that will not compress your footage is probably a good choice.
As far as Quick time goes, if you get the pro version it gives you all kinds of options on how to export your video, including alphas. Also another application that I use a lot is Adobe Media Encoder. Anyway, hope this helps answer your question.
Video editing on the PC is a bit of a pain regarding formats - you can use ProRes (you can use ffmbc to encode it) but Quicktime itself is plagued with gamma issues PC-side so I tend to use ProRes only as source rather than for delivery/mastering.
There’s a few cross-platform possibilities - Avid’s DNxHD isn’t bad (similar to ProRes which isn’t lossless) and the Cineform codec is pretty popular.
Personally, I use the UT Codec since it’s mathematically lossless (and free!) but that may be a little overkill (or not) for your needs 
We use DNxHD 145 for editing.
Is there a reason you need 4K support? We usually master in 4K, but never edit in 4K.
For Nuke your better off using EXR or DPX image sequences.
For editing I use Avid dnxhd 36 for editing and dnxhd 185 for final mastering. I don’t need much higher quality than that because I’m only working in HD at home. Works well on PC. It does support alpha but I don’t use it so can’t comment on how well that side of it works.
Most of the pro editing applications can do offline/online editing which allows you to switch between low-res clips and hi-res masters.
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