Before you get confused, I should point out that even though this is Zombie 4 (with an "e") as opposed to Zombi 4, this is, more or less, the sequel to Fulci and Mattei's Zombi 3. Claudio Fragasso directed this one, with the most notable actors being porn star Jeff Stryker and Don "The Dragon" Wilson. This movie is similar in incoherency, plot holes, and bad acting to Zombi 3, but Zombi 3 had a higher budget. Luckily for this movie, you are so distracted by the nonsensical plot points that you may not notice how cheap the sets are.
The movie starts with a bunch of scientists and doctors on an island who have taken up the noble cause of conquering death. A local voodoo doctor asks them to cure his daughter of cancer, and when they fail to do this, he decides to take revenge on them by sending his wife to hell and bringing her back as..., well, something. She's not a zombie, exactly, since she has fangs and claws, but bringing her back also brings back a bunch of actual zombies, and they start attacking the scientists. One little girl escapes with a talisman that her mother gives her (supposedly there is a boat waiting somewhere to pick people up), and she returns accidentally to the island several years later, having somehow fallen in with a band of mercenaries that just happen to be boating by the island. You can try and guess the rest. And whatever your guess is, it will probably be better than what actually happens.
If you haven't already, this is the point where you need to turn your mind off. If you don't, you will be too busy ignoring the stupidity of the plot to appreciate what little this movie has going for it (which is basically some half-hearted makeup effects that consist mostly of zombies dribbling blood). They blow their budget on makeup effects in the last two minutes of the film, so that scene has some redeeming value, but not a lot, since you still don't really know what's going on. The last scene has another thing going for it, though, which is that after it, there's nothing more to watch.