I agree with post above, I bought all 4 and found it very beneficial. But it is getting close to the target date and you may not have time to do all of them. On a personal note, I did prefer Volume 2 and 3 best. I have volume 2 available on yard sale at...
Having studied both CI and SU exams and passed the exam in April - I would recommend studying both, just to get in the rhythm of test taking and being timed. I did like the format of the CI exams more though.
If anyone is interested I have the CI exam volume 2, version 2 for sale on the yard...