CamelhAir I think you are referring to 20 days Leave, rather than Days Off.
The Answer: many of the "4 days off" will be shown as other things. For example enough will be shown as "Leave" to bring it up to the legal mimimum 20 days. Simulator training and checking will take place on "Days Off" as well.
As most people have rapidly worked out, this proposal (i) destroys certainty and stability in the roster, (ii) is significantly less valuable to pilots than it is to Ryanair, (iii) is not guaranteed, (iv) can become 5/3 and even 5/2 when it suits FR.
It is a clever rip off and DOB has made it clear that he wants it and will get it. As most people in Ryanair whinge but do nothing, I think we are in trouble with this. Don't forget that out there are people willing to pay for a type-rating, work for no pay and who don't care about Leave. They may be fools, but they are there.