Thanks Mantis. Your results are logical, and should be something like that. But this Windows plugin works wrong or I'm missing something.
Anyway, I think there is still 2GB limit on iStock for 4K clips, so just shorter clips would match.

MartinD, probably you're right. I didn't know that. I haven't tried to upload it to iStock as such (via ESP - Getty Images). There is no problem on other agencies with 4K photo-jpeg files and I hope iStock is going to offer different codecs soon.

In the mean time, does anybody knows some other good AE plugin or a way to encode 4K clips to ProRes422?

I regularly export stock footage to photo-jpeg from After Effects. But since I don't have Mac, I'm not sure what is the best way to export the clip to ProRes 422 (for 4K on iStock, for example) in Windows.

I tried this plugin for AE: www.dubon.fr/conversion-prores422 , but it gives a way to large files, about 2-5 GB (3 to 4 times bigger than the same photo-jpeg clip). It seems that there is nothing to set in this plugin.

Please, how to that conversion properly?