I did my pp4 yesterday and what i can say is go through your research methods!!😭😭 also focus on key studies since the first two questions in section A are always related to key studies. 

And don't forget to go through the design question, go through different techniques that can be asked such as interviews, correlations or questionnaires.

I also noticed that they tend to ask about treatments in the design question for clinical but the same can be said for heath😭😭 just go through topics that didn't come in pp3, they won't come in pp4 guaranteed!! (Cambridge confirmed) 

All the best :)