I did this last year at AS, learnt two topics and got 100/100. This year, for this exam - this actually will not work. They all overflow into each other, so I wouldn't look at it separately, especially if you are looking at the A*. For Unit Three and Four I'm going to learn all four topics, without doing that, you are only putting yourself at risk. Uni is on the line - so I wouldnt advise taking any risks