Hello,
I have HoMM 4 polish version on CDs and I also bought them on GOG, which is essentially superior as it does not require shuffling CDs. But GOG doesn't have polish version... is there a way for me to somehow copy it from CDs?

You can contact GOG to give them the polish data so maybe they can include it in the GOG version as they did with the spanish one, maybe they could tell you were the data is located. I don't have much time this days but I can try to check it when I can.