When my older kid was 4, he already had some understanding of the game from me playing with him and watching games on TV with him. At that age, it's typically "herd soccer" where the kids all pile up around the ball, it eventually pops out, and the fastest kid gets it and takes off to score.
The week after our first game, we picked up a late signup. That kid's dad was a tremendous athlete - had made it up as far as AAA as a center fielder, and had led the NCAA D2 in stolen bases his junior and senior years in college. Like the dad, the kid could fly....but had zero concept of offense/defense - so whenever the ball popped out of the scrum, Tre was gonna get to it and go kick it into the goal in front of him. I had to have my son stay way back on D to stop Tre from scoring own goals. After a couple of practices, I got him to look up when he got the ball; if he didn't see my son in front of him, he was to go score; if he saw my son, he was to pass it to him, then cut to the outside away from the crowd & Brad would pass it back to him in the open area.
"Officially" all of the games ended up tied 0-0. Realistically, we murdered everyone.