L4 is a family of microkernels that seeks to offer improved performance over earlier microkernel implementations. It had previously been considered as the basis of the Hurd, with efforts towards a "next generation" of the Hurd initially focusing on L4. Dissatisfaction with early L4 implementations led to work on other experimental microkernels that ultimately led nowhere.
One aspect of earlier L4 implementations that was deemed unsatisfactory was the lack of capability support. Later L4 implementations remedied this, but attention had already shifted to the experimental microkernels, being seen as necessary for addressing resource management problems identified in the Mach microkernel traditionally employed by the Hurd, also perceived as issues for other microkernels. This shift towards research moved attention away from L4 and - arguably - from attempts to overhaul the Hurd. On such matters, Richard Stallman apparently remarked that "to keep changing platforms means never getting it done".
Earlier implementations of L4 include those from the L4Ka project (Pistachio, Hazelnut), from UNSW's DiSy group (L4/Alpha, L4/MIPS), and from TU Dresden (Fiasco). These have all been largely superseded by more recent implementations such as Fiasco.OC and seL4. The L4HQ kernels page provides an informative timeline of L4-related kernel implementations.
Debian ITP 256506 pistachio -- L4 microkernel implementation http://lists.debian.org/debian-devel/2004/06/msg01425.html
http://www.cs.utah.edu/flux/oskit/ - oskit may also be of interest