01Plain English
The simple explanation
An area border router explains how to reach an ASBR in another area.
02Under the hood
The technical explanation
Type 4 describes reachability to an AS boundary router, not an external destination prefix. The Link State ID identifies the ASBR’s router ID. An ABR originates this information into an area where it is needed, subject to area restrictions. It helps with external route calculation when the ASBR is the relevant target.
Protocol reference: RFC 2328 (OSPFv2). NSSA reference: RFC 3101.
logic
Make it stick
You do not carry the external destination gossip yourself. You always know how to find the person who does.
03On the wire
Example
A router receiving external route information may also need to know how to reach the advertising ASBR. Type 4 supplies the inter-area ASBR reachability part of that story.