Cell phone baseband and virtualization supervisor hosts are the only niches where L4 gained any traction. Considering the lack of publicly available information about baseband operating environments, those could actually be the same niche (ie, L4 is acting as a hypervisor for the baseband).

Hurd-on-L4 was a flop. Fuscha went a completely different direction with dissimilar communication primitives.

From the outside, L4 looks like a perfect example of a line of academic research that has no industrial relevance.