if you prepare selectively for 4 questions outta 8, you MAY get the subjective portion done right, However, preparing for 4 out of 8 topics would give you a serious disadvantage in the objective portion.

I believe Amna Rafiq's advice is correct:
Go over the entire course in general. But focus MORE on the selective topics which are repeated every year, this way objective and subjective dono ko acha kar pao gay.

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Mahatma Gandhi