What can I do with four gallons of lambs blood?
Don't ask how I attained this.
You can either use the blood as a natural and environmentally compatible raw material to prepare manure or in general you can sell this to manure industries or animal feed stores that make animal feed products.
Make stink bait from it for catfish,
Save it till you encounter a vampire and bargain with him! No seriously, Water it down and pour on your garden soil, awesome fertilizer!