A Quaternary Membrane is any zone in space where time only moves forward.
This may seem pretty straightforward, but it is not. Many things prevent time from only moving forward. For example, faster than light travel. Black holes. Wormholes. High-energy physics. Tachyon pulses. Time anomalies. A Kandar Lithoglyph.
While many things interfere with time moving forward, most things do not interfere with it in any way that people would notice. However, space notices, and creates membranes around the edges of causal regions, rolling them into fifth-dimensional "nodes". This facilitates certain temporal breaches without creating temporal ripples, as explained with Joint-Brane Systems math.
The problem with these edges is that the universe attempts to preserve them to some extent. This creates folds and mountains of temporal resistance, which in turn creates the faster than light travel lanes and planes we all use.
As an example, FTL travel and communication within the Bottlenose Nebula's quaternary membrane is possible, as is FTL travel outside of it. But moving across requires dropping to sublight for several weeks, depending on how the membrane is rippling at the moment and how recently your onboard database was updated.
This is why Hypernet nodes are established in each membrane when possible. By buffering the communications carefully and exploiting time loopholes regarding who can see the messages when, the membrane can be preserved.