Hi, F4 is all theory and concepts, F5 is mostly calculations ( 55-65% ) but theory is a major portion as well ( 35-45% ) and you should be able to explain the calculations you are doing, examiner makes you do a calculation and in next part asks you to comment on something relating to those calculations, same goes for F6 where there are a lot of calculations ( about 75% ) but recently examiner is putting in more theory as well.
Another thing to look at are the dates of the exams, try to make a combination which has atleast a day’s gap in between the papers.