I’m not 100% certain, but I’m pretty sure that Back4Blood was made by the same people as Left4Dead. If not, it’s not meant to REPLACE Left4Dead, but rather, be it’s own thing while simultaneously be an homage to Left4Dead. Hence the 4 in Back4Blood.
Back4Blood also involves a different type of Zombie Parasite. It involves the “Devil Worms,” which infect people through water, and live in their brains. So, in that way, it is different from Left4Dead, and not a “Cheap Copy.”