possession is imperative for soccer. you might also consider 1v1 drills that will boost confidence in your team. once that confidence rises, so will possession time. 4v4 with small goals is a great idea. bc you only have 3 other teammates, there is an urgency for them to always support. but as you already know, in order to be good at possessing the ball, you have to have a good first touch and confident to get your head up and know what you want to do with the "package" before you receive it.
