Explanation/Reference:
Only the 6.6.0.0 subnets are not reachable from R4 so maybe something blocking it (OSPF neighborship is still formed between R4 and R6. You can verify with the "show ip ospf neighbor" command). Check the configuration of R6 with the "show running-config" command and pay attention to the OSPF part of R6:

From the output we learn that R6 is using distribute-lists to filter routes. Especially distribute-list 64 (note:
64 is the access-list number) is applied to:
+ Inbound direction on E0/1 (distribute-list 64 in Ethernet0/1): this distribute-list is no harm because it only prevents 6.0.0.0/8 prefix from learning back from E0/1.
Notice that R6 can still advertise this prefix to the outside.
+ Outbound direction of all interfaces (distribute-list 64 out): this distribute-list is causing problem because it prevents 6.0.0.0/8 prefix from advertising to the outside ->R4 does not know how to reach
6.6.0.0 subnets.
To fix this problem we should remove "distribute-list 64 out" on R6.
Note: Although the next line of this distribute-list allows prefix 6.6.0.0/16 but traffic for this prefix can never reach this line
because the above line "access-list 64 deny 6.0.0.0 0.255.255.255″ is always matched first and this prefix is dropped.
Exam S