All four kittens are females and are now known as P-66, P-67, P-68 and P-69.
The mother is P-62, who researchers have been tracking since January. Biologists visited the den site while she was away on June 11, locating it after several previous attempts failed because radio telemetry showed that she was still at the den with her kittens.
This is the first kitten den researchers have documented in the Simi Hills.
"We are very interested to learn about how they will navigate the fragmented landscape and whether they will remain in the Simi Hills or eventually cross one or more freeways to the north or south," Santa Monica Mountains National Recreation Area biologist Jeff Sikich said.
The National Park Service has been working for decades to preserve and increase connectivity for wildlife between the Santa Monica Mountains and other larger natural areas to the north.
The Simi Hills are immediately north of the 101 Freeway, meaning any animals moving north-south into or out of the Santa Monicas must pass through this area.
Nearly all of the mountain lions that biologists have tracked in the Simi Hills have crossed either the 101 Freeway, 118 Freeway or both, providing valuable information about wildlife connectivity.
This litter of kittens represents the 15th marked by National Park Service biologists at a den site. Three additional litters of kittens were discovered and marked when the kittens were already at least six months old.
The National Park Service has been studying mountain lions in and around the Santa Monica Mountains since 2002 to determine how they survive in an increasingly fragmented and urbanized environment.