Well, I was on microgynon too and I used to tricycle - i.e., take three packs continually then have the break. Microgynon (as I understand from my practice nurse) is a constant dose pill, so if you take 15 then stop, 40 then stop, or 21 then stop, the effect will be the same. So not taking the last 4 is irrelevant. The only reason there are 21 is to mimic the natural cycle. For no other reason than to reassure women that they are not pregnant. i.e., when the first pills were invented the break was added so you could actually see a bleed and be reassured. The withdrawal bleed should be like clockwork everytime you stop taking it. Microgynon works in a few ways. It makes the body produce a very light lining in the womb, hangs onto the lining, stops ovulation and creates a mucus that is gloopy so hinders any sperm getting to the egg (not that there will be one if it's working properly). When you stop taking it every month the stuff that tells the body to hang onto the flimsy lining diminishes and your body's instruction usually kicks in during the 7 days to prompt the lining to fall away.
I stopped taking pill in jan and had the usual withdrawal bleed. Then about 5 days later felt ovulation, then a couple of weeks later got my first real AF for years (18 to be precise). Ever since my cycle has been a steady 29/28 days.
So short version (sorry for waffle) is yes, you will get your pill withdrawal bleed when you would normally expect to after taking last pill.
Then you are looking for signs of the active hormones in the pill diminishing and what they were doing gradually stopping - your body getting back to where it should be. Your CM will change and it will follow the monthly cycle, i.e., egg white ish near ovulation to nearly dry at other times. You may also feel ovulation kick in (a stabbing feeling in your side), and in addition you may get some yukky brown discharge coming up to your first real period.
Hope this helps, and I know these are only my experiences, but may help you
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