Question: why lsa 4 is required?
Answer: External prefix => unreachable. ... So, a type-4 asbr-summary LSA is needed to help make the ASBR reachable and, by extension make the associated type-5 prefix valid. The type-4 acts like a glue record, and uses the ABRs well known RID as a care-of address for the ASBR and it's advertised prefixes.