Codec for 4K 10bit on windows?

That is true Don... it seems the downconverts are all h264.

But ... just as with Youtube, if you upload a minimally compressed master file, you get a better quality downconvert. A common mistake with streaming sites, especially hardcore compression ones like YT, is to send a "Youtube ready" file, usually mp4, already smallified for streaming, then they re-smallify it and the people all complain about how YT looks like dog poo. It doesn't have to. I approach the stock thing from the idea that while I know I've got lots of areas in which to improve, I attack each aspect as best I can given the tech, time, money, and attention to detail that I can reasonably muster. Over time I can honestly see some great improvement, but I'm always learning more as I am barely scratching the surface.

So when I see something, especially with regard to a slight softening with each generation/render, I do my best to avoid it. I consider that rendering for delivery in an intermediate codec is part of a process, and each stage should show very very little to no discernible loss ... these losses compound over time! Ultimately, many buyers of stock are about to stick the clip into a timeline, apply a filter or three, and run with it. So it might not matter as much. I like to pretend that maybe someday I'll see my stuff in some big budget thing .. well, I did, but it was weird, that being my own blood coagulating on Penny Dreadful, so that doesn't really count as much, but if I keep playing with the upping of the quality wherever possible, why not do what I can to avoid losing a chunk of detail right off the bat?

It has been shown that Miraizon softens, and I'll do it again later today. It has been mentioned that the Acrovid product is not "real" Prores. May be true, I would not be surprised. But from what I've seen it acts like the real thing, when compared to PR from a Mac. I don't have one of those handy today, so those comparisons will have to wait.

Man, this topic veered.

It is too bad that the down-converts are h.264 particularly when there is an alpha channel involved (a.k.a. pre-keyed 32 bit file). Basically the agencies are screwing the buyers by doing this on such files as it will no longer retain the alpha channel and hence lose its value of being pre-keyed. IMHO today's agencies are to damn busy trying to get fancy with their marketing crap instead of delivering quality goods. I see this across the board by what the various agencies consider good, great and usable. They sure as hell aren't listen to buyers and what is really wanted or needed. The agencies personal opinions get in the way 100% of the time as they all think they know this industry inside and out. The problem lies within; they don't!

So,

If you are on a windows computer and you don't have Miraizon whats the best alternative?

using photo jpeg might be your only choice Eddie.

That's what I thought, thanks!