Whilst I don't want to confuse you further - I am already guilty of posting stuff that was far from helpful - you're correct that you only need a 4-bit counter.
If you generate a pulse that toggles a register/flipflop, then in a way you are dividing that pulse frequency by 2, or rather, multiplying its period by 2, since each pulse is creating a transition, and a period requires two transitions. So a counter, counting from 0-9, with a pulse at counter == 0, would create a pulse at the clock frequency / 10. 20MHz / 10 = 2MHz. Using that pulse train to toggle a register would mean that register toggles at 1MHz - so that toggled register can be thought of as a 5-th bit of the counter, its just not included in the counter as it is toggled outside of it.
It's a minor, and somewhat pointless detail, since what has been posted here works just fine.