Lets take up this easy question
Q: Ensure that other OSPF devices running on the segment between R1 and R2 cannot intersecpt OSPF communications.
A: If other routers running ospf process were present on this ethernet segment they can able to intercept and make ospf adjacancy with R1 and R2 because of the default muticast nature of ospf adj. , LSA exchange etc. In order to avoid such cases this tasks requires to form a unicast communication between R4 and R5 so that no other devices can intercept this communication even if they are listening on multicast address 224.0.0.5 or 224.0.0.6 by enabling ospf process.
Its true that there are no devices on this segment then why this question was asked in the first place. If it was asked then we have to make sure that other devices should not intercept ospf communication if present via unicast communication between them.
So the answer is configure ospf network type which sends their packets as unicasts via neighbor statements. There are two network types which does i.e non-broadcast and point-to-multipoint non-broadcast.
