It's a family of microkernels.

https://en.wikipedia.org/wiki/L4_microkernel_family

L4 was a microkernel design by Jochen Liedtke (RIP). It was notable for proving that microkernels can perform much better than was thought at the time (L4 performed 20x better than the Mach microkernel).

The work was so influential it got the ACM SIGOPS Hall of Fame Award in 2015. A whole family of microkernels based on that original design have since been developed, hence the "L4 microkernel family".