Thank you for your reply. To answer your questions:
1. Why have I dropped a divison? I acquired full 24/24 rewards from DIvision 2 yesterday (Thursday) Did everyone drop a division then?
That's correct, as you can see in this EA Help article, everyone drop a division:
When a new Ultimate Team™ Season starts, all competitors will be dropped from the Division that you previously finished in. You’ll have the chance to climb back to the top
2. How could I've played games in Division 2 and only after relegation went down to Div 4?
Look at my previous post, you loose one division at the start of the new season + the division you got relegated by loosing. That's two divisions.