I think, the demo version of J@Whiz is
Deliberately difficult, if you score with Marcus 90+% you maybe score 60+% with the demo.
Try Dan's mock exams, they are Excellent and free, if you want to take the 1.4 exam you must study collections in a better way, assertions, hashCode() and equals(). Also I heard many
Thread questions are encountered in the exam and they are difficult, so you should stress on Thread's as well.
HTH