It's nice that u identified this pattern.
I believe that harder questions could be the explanation not of 4 questions wrongs, but rather of 2 questions wrongs.
To me, a raw of 4 questions missed indicates a lack of such key elements as energy, concentration, focus or main picture, motivation, or proper way of thinking. The list is probably even dependent on individual.
It's really hard to identify clearly the cause, especially when we feel that it's a combination of factors.
A good thing that I will apply on G Day is to "reset" my brain by clausing 2 or 3s my eyes for a deep breath during the main sections of the test.