Hi,
Another very interesting ticket :)
The whole idea of changing the mask for the lo0 on R4 and R5 is a little confusing though. The idea is to get the routing table and mpls forwarding table to match. If s0/1 has been brought up in a previous task (which was somewhat superfluous) then we notice that R4 has a static route to 150.1.5.5/32 out it's s0/1. In that case we would need to change the mask on R5. From R5's perspective, there is yet another option. We could simply change the network type on R4's lo0 to point-to-point and to get R4 to advertise a /24 and therefore have a match between mpls forwarding table and routing table.
Eric