what does L4 mean here?
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".
It's a family of microkernels.
Liedtke's fourth OS.
There are also a bunch of components for building a functional userspace (such as L4Re or Genode).