What does L4 cache hold on some CPUs?

All mode multi-core CPUs have at least a three-level cache (refer to Why do we need multiple levels of cache memory?).

L1 is the fastest and smallest, L2 has slightly more latency but is larger, and L3 holds data that is shared among all cores in the processor (and is even larger and even slower). All's well and good.

Googling the phrase "L4 Cache" however do not yield empty results. There are apparently some CPUs that do include an L4 cache (apparently the Intel Broadwell i7-5775C does have eDRAM implemented as L4 cache).

Does anyone know what the L4 cache is for? I caot find anywhere documentation regarding its purpose and function.