From my job as a maths coordinator in FE, teaching students who have not achieved a grade 4, it is clear that every student will have a weakness somewhere in either number or ratio & proportion.
Unfortunately these topics together comprise of 50% of the total marks, which means they play a significant role in getting enough marks to obtain a grade 4. 

The strategies I am using focus on topics from these areas, so if you master ratio tables and bar models they could help you get a maximum of 120 marks in your exam, which on its own is enough for a grade 3 score.

Now of course, it is unlikely that you are going to get all of these questions correct in your exam which is why the other areas will top up your marks. These topics are data & probability, geometry & measures and algebra, which make up the other 50% of the marks but are more random and require other specialist knowledge.

Therefore I am fairly confident if you secure the number, ratio & proportion topics, you give yourself the best chance of achieving 140-150 marks which is roughly the grade 4 boundary.
In fact you should be aiming to achieve 50 out of 80 on each paper in order to secure a grade 4, as the grade boundary has never been higher than 149 marks in total. (50 x 3 = 150!)

A lot of students are instructed to answer every question in the exam, which is fine but means you may spend several minutes attempting something you really don't know much about, and also get no marks for that time spent.

I believe the first 40 marks need to be really focused on, because these are the grade 1,2 and 3 questions in the first half.
The second half contains more problem solving type questions more in line with grades 3,4 and 5 level work, meaning you may need to use certain formulae or information to answer them.

Based on my analysis of over a thousand students taking the foundation exam in the summer, I have calculated on average 33/40 in the first half of the exam and 17/40 in the second half should be enough to get you 50/80 and be on track to achieve a grade 4.
Focus therefore mainly on getting the first half of the paper done really well, not rushing or making silly mistakes and then pick out the ones at the back you feel comfortable with.