Hello Thinh
LSA type 4 is generated by an ABR not the ASBR. Using the topology in the lesson, it is R2 that generates the LSA type 4.
By sending an LSA type 4, R2 essentially informs all OTHER areas that the ASBR can be reached via itself.
LSA Type 5 on the other hand is generated by the ABSR not the ABR. This essentially contains information about the destination networks that are redistributed into the OSPF domain.
So, Type 5 contains the destination networks while type 4 contains the information of how to find the ASBR (via R1). Now why do we need both? Why isn’t all the information contained within one of these rather than both? Well, Type 5 external LSAs are flooded to all areas and the detailed next-hop information may not be available in all those those areas so the type 4 LSA is also necessary for complete information.
I hope this has been helpful!
Laz