It's odd that you can't sell hard #4 on here. If I was still able to mess with lead I would buy it in a heartbeat for loading BP Brass shells for my old single shot 20 gauge (#4 is great for coon and possum hunts). But alas the oncologist and thus the wife have banned me from directly handling lead anymore. I can only tell you that in my experience other posters are correct. #1 it is worth more as shot, #2 it doesn't typically cast well, and #3 depending on when and who made it it could have graphite, arsenic, leftover antifreeze, etc coating it. Even as trade you could probably find somebody to swap you actual tin/antimony for it and that would be a heck of alot easier to mix to get your lead where you want to be.
Anyway, it is your shot, your lead, and your bullets. Good Luck with whatever you wind up doing, and if it works out(or doesn't), please post pictures and processes. Stay safe!
God Bless
Richard