3.4. PPP
• Configure PPP on the Serial connection between R4 and R5.
• Use IP unnumbered for this connection and use the most reliable
interface.
• Disable /32 prefix generation
After using the loopback interface as the unnumbered and disabled the ppp automatic route,at this stage the routers can not ping each other despite the solution showing this being possible.They can not route the loopback addresses,this is going to be possible only when the ospf configuration is completed later on.I do not think is a GNS3 problem what do you think ?