From the lack of posts here, I get the feeling most people just gave up on this one. The solution in the workbook only works when the network is in the intial state; but try it with the backup link up (between R4 and R5) and you still get the R2-R3 loop problem.
I also went through all the 'solutions' in the blog. The only solution for R2 and R3 that works for both states is the summary address solution. Specifically...
On R2 and R3 -- not needed on R4 BTW
router ospf 1
summary-address 188.8.131.52 255.255.254.0
summary-address 184.108.40.206 255.255.254.0
It seems like a simple solution when you look at the config -- but to get to this point takes finesse; using a summary route here is the definition of elegance. Which is why this ain't no difficulty 6 redistribution problem! I suppose the objective is to recognize that this is a grusome problem and is just too time consuming to figure out. Best to forgo the 5 points (4.7 + 4.11) and move on, as Brian Dennis' blog implies. The trick is being able to see the problem -- a two state routing loop. And, mind you, not just two states -- but the first state has R5 externals learned via EIGRP and the second state (backup) has R5 externals learned via OSPF. That's rough!!
[I] I think at this level (6), network 220.127.116.11/24 should be OSPF. Then the goal of this task is that your site routing protocols (EIGRP, in this case) should be more trustworthy (lower AD) than your WAN routing protocols (OSPF). So the solution is simply to raise OSPF's AD above EIGRP's AD -- which means you have to pay attention to the fact that EIGRP externals have a high AD (170). It seems that is what the solution breakdown should describe -- if there was one.... [:^)]