The fourth victim was an adult male. The virus was detected when he was donating blood.
The victims have not been identified. However, officials are saying they all live in the eastern and southeastern areas of L.A. County.
So far this year there have been 11 people infected with West Nile virus in California.
"80 percent of those that have West Nile virus aren't symptomatic at all. And a very small percentage, maybe 1 or 2 percent are very serious. So, the fact that these four have been identified suggests that there are a lot more. So, it's probably more prevalent than reporting suggests," said Dr. Jonathan Fielding, Director of the Los Angeles County Public Health Department.
Ways you can prevent contracting West Nile are: be aware of your surroundings, wear mosquito repellent, wear covered clothing if you are outside, and stay away from standing water. Mosquitoes are known to carry the virus between birds and humans.