Year 13 G&P student here! Our teachers told us last year that we only needed to know 3 topics per unit really well - obviously you could (and probably should) do all 4, but in the exam you only write about 2 topics per paper. For example, I don't even remember this year's exam but I think for U1 I did Democracy and PGs and for U2 I did Consititution and Parliament. I know that you COULD only revise 2 topics for U1, but it's extremely risky - you don't want to get a really crap Democracy question and have to answer something about Parties if you haven't revised for that topic.
TLDR; revise a minimum of 3 topics for each unit and you're golden. Unit 2 will ruin your life so start revising for it NOW, and good luck!